Yamimash

Aaron Ash, better known by his online alias Yamimash or Yami for short, is a British YouTuber who has played multiple games with Markiplier before. Both of them have played Gmod horror maps and a Slender multiplayer game, one of their most well known collaborations. On the channel, Yamimash was one of Mark's most loyal and greatest companions next to Bob, Wade and Jacksepticeye. Yamimash resides in Warwick, Coventry, England. Mark refers to his home as "Britonland".

Collabs with Markiplier
Markiplier and Yamimash have played multiple games during their collaborations.

Slendytubbies
Markiplier and Yamimash have both played the multiplayer mode on both Slendytubbies games. Mark would play as a slendytubbie and lay out the tubbie custards to be collected while Yami would go around and he would try to collect them.

Gmod
Markiplier and Yamimash had played on Gmod horror maps together as well. In Gmod, they would go around and see what horrors awaited them and at one point they did it wrong in one episode simply titled Garry's Mod Gone Wrong. Yamimash is the main guest in all of Mark's Gmod videos and appears in nearly every one of them except the Prop Hunt videos. He's most prominent on the GMOD Videos.

Drunk Minecraft
Yamimash had appeared on a couple of episodes of Drunk Minecraft. His Minecraft skin is revealed to be Ash from Pokémon and he participated in the Minecraft Hunger Games episode. He's only appeared in a couple of episodes though and isn't a prominent guest.

Trivia

 * Yamimash has played a "hacked" version of Five Nights at Freddy's 3. However, the game turned out to be a fake, made by Scott Cawthon himself. The content was copied from Scott's older game "There is no pause button" with Freddy's head replacing the sprite's.
 * Yamimash has multiple fake accounts stalking and hating on him on almost all his recent videos. These accounts usually go by the name "Mel Gibson" or "Gel Mibson".
 * Yamimash and Markiplier have a popular pairing called "Markimash".
 * Yamimash's passion for playing Pokémon titles has been used to mock his channel by other YouTubers. Bob seems to poke fun at this the most.
 * Yamimash's girlfriend considers herself a big fan of Mark. She has played games with Yamimash on YouTube and appeared in a collaboration using a microphone to politely say hello, especially to Mark.
 * Yamimash met Markiplier in person for the first time at PAX in early 2014. It was recorded by Yami and is featured on his channel. Mark featured some footage of the two battling like American Gladiators at a PAX padded battle arena.
 * Through their adventures Mark and Yami have formed their fair share of inside jokes. Two of these are related to Gmod. "Colon Blow", according to Mark, "laxative with explosives" is their nickname for grenades. When they have to reach each other from a spawn, are haphazardly skipping puzzles, or observing whats left of a map, they will No Clip while mimicking flatulence noises.
 * Mark's impersonations of Yamimash's English accent can be as numerous as they are different. Mark's attempt on one particular occasion was reminiscent of actor Michael Caine's portrayal of Alfred from Batman. This often resulted in reverting to Batman character voices. At times, Yami and Mark use other comical or faux voices together, most of which could never be pinpointed.
 * Yamimash occasionally refers to Mark as "bumbum", a name Mark has used before in games, namely the rage platformer Bloody Trapland.
 * Mark is well known for an altruistic attitude towards boxes - unless they're involved in a stacking puzzle and in Gmod with Yamimash. Out of on occasion nowhere, Mark will triumphantly yell "BOXES!" while destroying every box in the room. Manifesting with Yamimash's amusement and Mark's bewilderment, they discovered that progress involved boxes that were usually indestructible at times.
 * Contrary to the rumors, Mark did not have a fallout with Yamimash which people believed to be the reason why they no longer collaborate with one another in their videos. Mark debunked these rumors with the real reason being their timezones as Mark stated in the first episode of Don't Starve Together that due to their respective timezones they cannot create a proper schedule for them to work together in a video.