Tailgate

Tailgate was Arcee's partner on Cybertron during the Great War. He was captured and killed by Airachnid, which led to Arcee's shellshocked trauma.

War on Cybertron
Tailgate was a member of the Delta Team, along with Arcee, on Cybertron. On one combat mission, Tailgate failed to take down a Decepticon sniper, forcing Acree to attack the Decepticons directly. As the two discussed where to meet, Acree was captured by Airachnid. While Arcee was captive, Tailgate was captured and brought to Airachnid.

Airachnid then killed Tailgate, after Arcee stated she didn't know coordinates of an incoming Autobot attack.

Arcee was then rescued by Cliffjumper and Bumblebee, but she would never stop grieving for his death. When being saved, Arcee said she couldn't save her partner.

After Death
Years after his death, Tailgate was still remembered. When Cliffjumper and Arcee were captured on a Decepticon warship Arcee refused to work with Cliffjumper, fearing that he might suffer the same fate as Tailgate had. Cliffjumper then told Arcee he knew what happened to Tailgate, as he was part of the rescue mission that saved Arcee.

When Cliffjumper was shockingly killed, Arcee had suffered the loss of another partner, the first being Tailgate.

Airachnid also constantly reminded Arcee of his death in attempts to intimidate her.

When Starscream was an Autobot prisoner and him and Arcee were fighting, Starscream said: "Was Tailgate weak like Cliffjumper?" which enraged her.

Arcee said to Smokescreen to keep on his guard because, she had already lost two partners, one being Tailgate and other Cliffjumper.