In 2258, in the alternate reality, Keenser was working with Montgomery Scott at an automated Federation outpost on Delta Vega. He was Scott's only company for months before Spock and James T. Kirk arrived.
Scott often belittled and berated Keenser, particularly regarding Keenser's tendency to climb up on things and the fact that his species required very little food. It was clear that they were fond of each other, however; Keenser, in particular, was saddened when Scott transported with Kirk to the USS Enterprise.
A year later, Keenser left the Enterprise with Scotty when he submitted his resignation; he had objected to not being allowed to examine a group of top secret photon torpedoes, which had been brought aboard the ship and were to be used for the targeted killing of traitor and mass murderer John Harrison.
While Keenser was dining with Scotty in San Francisco's Port of San Francisco bar, they received a call from Kirk, who had opted to arrest Harrison instead, and received coordinates from him. Keenser glared at Scotty, silently insisting that he investigate the coordinates.
Keenser was still on the Enterprise when it was destroyed during the Battle of Altamid. Though suffering from a bad cold at the time, one which Scott feared might lead him to sneeze on the warp core and cause a warp core breach, Keenser helped Scotty hook the core up to the impulse engines and was captured by the forces of Krall during the escape from the ship. Keenser later used his acidic sneezes to briefly break Uhura and Sulu out of confinement, but they were later recaptured. Along with the rest of the crew, Keenser was rescued by Kirk, McCoy, Spock and Jaylah. Keenser was beamed to the USS Franklin with the last batch of crewmembers where Scotty excitedly greeted him. Scotty nearly hugged Keenser but ultimately settled for a handshake. Keenser then helped Scotty repair the Franklin and worked in Engineering during the attack on Yorktown. Keenser later attended Kirk's birthday part and presumably joined the crew of the USS Enterprise-A along with everyone else. (Star Trek Beyond)
Background information Edit
Keenser was played by actor Deep Roy.
Much of the work that went into designing Keenser was done with sculptures of his head and shoulders. (Star Trek - The Art of the Film, p. 130) One initial decision that Director J.J. Abrams made was that Keenser's eyes shouldn't look totally Human. When Lead Creature Designer Neville Page was showing Abrams one of the early Keenser concept sculptures but was unsure precisely where the eyes should be on Keenser's face, Abrams told him that these features would stay in the same place but that, instead of having normal Human eyes covered by contact lenses, the idea was for Keenser to have a pair of strange, little, silver nubs that pop out of each eye socket. ("Aliens" special feature, Star Trek (Special Edition) and (Three disc Blu-ray)) In retrospect, Industrial Light & Magic Visual Effects Art Director Alex Jaeger commented, "Seeing Human eyes, it didn't look alien enough. To take it out of that realm [...] J.J. had described what he wanted as like the metal tip end of a pencil." (Star Trek - The Art of the Film, p. 131)
A total of at least four concept sculptures were crafted for Keenser. Additionally, a series of at least five concept sketches were done in pen, once the facial design was selected, to illustrate possible costume configurations. (Star Trek - The Art of the Film, pp. 130 & 131)
Proteus FX created Keenser as a full-head silicone makeup design. (Cinefex, No. 118, pp. 62 & 68) "We created all our original Keenser molds in BJB Enterprises' TC-1630 UltraCast polyurethane casting resin," recollected Prosthetic Makeup Designer Barney Burman. "They were very strong, and held up well." (Cinefex, No. 134, p. 82) The makeup was designed with the intention of featuring subtle CGI enhancement. (Cinefex, No. 118, pp. 62 & 68) Representing the eyes digitally was a very costly option, but it was thought that adding the metallic probes to Keenser would appropriately give him a more convincingly alien appearance. (Aliens featurette, Star Trek (Special Edition) and (Three disc Blu-ray))
Deep Roy was specifically sought to play this character. "I was introduced to J.J. Abrams by the art director. They were looking for me," Roy stated. "My good friend, Tommy Harper, who is the co-producer/production manager, was the one who recommended me. I was introduced to J.J. and he hired me, and that’s how it started." The role required two makeup artists to apply the makeup for approximately two hours each day. Despite Roy having been provided with his scripted dialogue, the filming crew chose to eliminate these lines during production. "They said, 'It doesn't look right for this guy to talk because, after all, he's from another planet. He's an alien. He doesn't speak the language.' So that's what happened [....] J.J. told me why and I said, 'Sure, whatever it is, it is.'"  Nevertheless, Keenser does have one line, when replying to Scotty's rhetorical question of whether any other starved and freezing Starfleet officer was in the room: "Me!"
Originally, Keenser was not going to join the Enterprise crew. However, Scotty actor Simon Pegg talked to J.J. Abrams about how sad it was that Keenser was left on Delta Vega. Abrams then asked Costume Designer Michael Kaplan for a small Starfleet uniform.  Recalled Deep Roy, "They were debating whether to leave me or put me on the Enterprise. J.J. created a special scene for me. I was finished with the movie, but I got a callback and they said, 'They want you for one more day.' They didn't tell me for what. They said, 'You'll find out when you come on the set.' When I went on to the set, J.J. said, 'We're going to put you on the Enterprise' [....] They actually made a Star Trek uniform for me, practically on the spot." Roy was delighted and grateful that the character was granted the opportunity to go aboard the Enterprise. "I said, 'Well, here comes the sequel. Maybe I'm a permanent fixture now.'" 
Following production on the film Star Trek, Digital Domain edited out the actor's eyes, created a dark cavity which was blended into the eye sockets, then added antenna-like rods that could dart in various directions. The animation of these rods required tracking of the actor's head and eyes to root the rods to the back of each eye socket and gauge their movements. Digital Domain Visual Effects Supervisor Kelly Port explained, "We tracked the actor's head and eyes separately, and then created a rig to align the tips of the rods with the actor's pupils. That gave us our first pass on the eye-line. Then we played with eye-stalk motion, adding little twitchy moments that resembled blinks." (Cinefex, No. 118, pp. 62 & 68) Roger Guyett, the film's visual effects supervisor and second unit director, remarked that, despite the resultant probes being a subtle detail, it made "a tremendous difference to the believability" of Keenser's alien nature. (Aliens featurette, Star Trek (Special Edition) and (Three disc Blu-ray))
For Star Trek Into Darkness, B2FX – the same makeup studio which had originally produced the Keenser makeup under the name Proteus FX – reprised the character's makeup. "Jamie Kelman and I applied the makeups with some subtle color changes," said Barney Burman, "but we kept as close as possible to the original." (Cinefex, No. 134, p. 82) Having the character not talk in the sequel "made sense," observed Deep Roy.  Nonetheless, off-camera alterations were made to the makeup to enable more flexibility of expression in dialogue. B2FX did this by creating facial appliances with three densities of silicone. "We first applied one very firm outer layer of skin inside the mold. We then added a thickened layer and a soft 180-percent deadened layer. That created rigid, shell-like outer structures, while the under-skin could flex whenever Deep emoted." (Cinefex, No. 134, p. 82) On a daily basis, the time it took to apply Keenser's makeup was now reduced to one hour and forty-five minutes, and only required one artist.  This time, the makeup was digitally enhanced by Pixomondo, adding the characteristic CGI animated eye-stalks to Keenser. (Cinefex, No. 134, p. 82)
When asked how he felt about the character following his participation in Star Trek Into Darkness, Deep Roy commented, "Keenser is a smart alien. I try to bring a little bit of comedy into it, but he's a great thinker. He is a [quiet] guy… silence is golden, as they say, but he's well aware of what's going on around him. He doesn't hold grudges against anyone. Even though Scotty gives him a hard time and always blames Keenser – whatever happens, it's Keenser's fault – Keenser just takes it with a grain of salt and moves on." Roy also expressed happiness that his work as Keenser was appreciated by fans and speculated, "Maybe Keenser can get his own spin-off, The New Adventures of Keenser. That could be a new franchise." 
A set of three concept sketches of Keenser can be found at TrekCore.
Simon Pegg once characterized Keenser as "an oyster-faced alien." ("Aliens" special feature, Star Trek (Special Edition) and (Three disc Blu-ray)) The magazine Cinefex (No. 134, p. 82) has similarly likened Keenser's distinctive eye-stalks to those of a crab.
The name of Keenser's species was not stated in either of the movies.
Keenser was the protagonist of the fourteenth issue of IDW Publishing's Star Trek: Ongoing, which identifies Keenser as a Roylan from the planet of Royla, where he lived with his parents. He was often ridiculed by his so-called "friends" because he was taller than all the others of his kind. In 2230, after first contact with his people was made by the Federation Starfleet ship USS Kelvin, Keenser demonstrated his engineering skills to the Kelvin's first officer, Lt. Cmdr. George Kirk, and security officer Lt. K'Bentayr by giving them a complete diagnostic and pinpointing the problem. With his parents' best wishes, Keenser was allowed to leave with the Kelvin's away team. He experienced space sickness on the way to the ship. He had graduated from Starfleet Academy in 2233, becoming the first Roylan to have left Royla and joined Starfleet, dedicating his service to the fallen George Kirk. The issue also identifies Keenser as having the rank of Lieutenant. His service on Delta Vega preceded Scott's, having outlasted all the other personnel formerly employed there.
In a parallel universe depicted in "Parallel Lives, Part 1", a female equivalent of Keenser, named "Keensera", is shown serving as a lieutenant aboard the Enterprise of that reality. Although Keensera herself does not talk in the comic, Marjorie "Scotty" Scott, the ship's chief engineer, mentions she initially suspected Keensera of causing, by messing about with the Enterprise's functions, entanglement in the ship's systems, an opinion Scotty implies she has since revised.
In "The Tholian Webs, Part 2", Keenser was rendered unconscious. After Scott went mad and tried to take over the ship, though, Keenser ended up saving the vessel. Captain Kirk subsequently noted, in a captain's log entry, that he wished to commend Keenser. They thereafter had a drink together in the ship's lounge and, although Kirk tried to make conversation, Keenser was extremely quiet, as usual.
He also appeared in the 2013 Star Trek video game, where he was established to be color blind. He plays an important role in the second half of the game; Kirk and Spock check out the source of a sound nearby, which turns out to be Keenser, who is cowering behind some storage crates in an attempt to hide from the Gorn. When they escort him safely to engineering, he is met with joy by Scott, who hadn't been sure the Roylan was even still alive. After performing a diagnostic on the Enterprise, he informs the player (through Scott) that the warp cores are offline. Kirk and Spock clear engineering of Gorn so Keenser can effect repairs. When Kirk shouts at Keenser to hurry up, Scott tells Kirk he doesn't have to be condescending, seeming to indicate that Scott has a low tolerance for anyone else insulting his friend.