Donovan is the team's technical specialist. He's an expert at hacking, repairs, and electronics. He is a smaller man, but his build is deceptive, and he can fight as well as the rest of the team.
One of the entries in Reilly's terminal implies that he frequently fights with Brick. According to Reilly's terminal, Donovan was also either a member of the Brotherhood of Steel or at least a contact of theirs before joining Reilly's Rangers.
After completing "Reilly's Rangers" you may encounter Donovan while traveling through the Capital Wasteland. Soon after he appears he should engage in a firefight against 3 super mutants (these can be brutes, masters or overlords dependent on level). When you talk to him, he will either thank you for your assistance (if you helped him kill the super mutants), lightly chastise you for just watching the fight (if you don't assist him or talk to him before the super mutants appear) or get killed by the super mutants. Regardless, he will give you a mini nuke and say that Reilly sent him (if he isn't killed; if he's killed, the mini nuke can be found on his corpse). After speaking with you, Donovan will make his way back to the Ranger compound.