Tailgate was the original partner to Arcee.
Tailgate was captured and was brought Airachnid, who had already captured his partner, Arcee. Airachnid was a Decepticon interrogator who need to know to Autobot's attack coordinates from Arcee. Since Arcee didn't tell, she killed Tailgate. Bumblebee and Cliffjumper rushed in to save Arcee and Tailgate, but were to late for him.
Even after Tailgate's passing, he is still remembered. Arcee has the fear of have partners since both Cliffjumper and Tailgate has died because of Airachnid and that her Human partner was almost killed by Airachnid.