Donnic is first encountered during Aveline's companion quest The Way It Should Be. Donnic will be one of the guardsmen who is set up by Jeven. Once the quest is concluded and Aveline is instated as the new Guard-Captain, Donnic lets slip to her that he looks forward to having Aveline "over me" before embarrassingly correcting himself to say "above me. In rank."
During the companion quest The Long Road, Hawke discovers that Aveline has taken a romantic interest in Donnic and is attempting to woo him with disastrous results. With some assistance from Hawke, Aveline can successfully enter a relationship with Donnic. The two later marry during the three-year interim period between Acts II and III.
Following the wedding, Isabela frequently teases Aveline about Donnic, whom she admits is "an incredibly proficient lover". He also apparently becomes friends with Fenris, frequently joining him, Anders and Varric Tethras for games of Diamondback at the mansion. The group, Donnic included, exclude Aveline because she is a sore loser.During the companion quest Favor and Fault, Aveline will assign Donnic to a dangerous patrol route in order to disprove rumors spread by former Guard-Captain Jeven that she coddles those under her command. Hawke and Aveline then help defend Donnic's patrol from an attack. The incident is submitted to Knight-Captain Cullen as evidence that Aveline is a competent leader and the templars have no basis for their attempts to gain control of the guard.
However, if you don't complete Aveline's quest, Donnic will be rude towards her during the entire game, and he'll have problems with her leadership as Captain.