Donovan is a member of Reilly's Rangers. He is located within the Ranger Compound if he survives the Reilly's Rangers Quest. If he survives he will repair you items at a discounted rate, lower than any other merchant in the Capital Wasteland.
If he survives the Reilly's Rangers Quest, the player may randomly encounter Donovan in the Wasteland. If you encounter him he will he engaged in a firefight with three Super Mutants. If he survives this encounter, he will give you a Mini Nuke and tell you Reilly sent it. He will then return to the Ranger Compound. If he dies, you can loot his body.