Cole traveled with Varric throughout those ten years, choosing him over traveling with Maryden. It was better to protect the world from the possible destruction Solas's plans would have on the world than to travel with a minstrel, bringing happiness and peace wherever they went. He could help more people this way, even if it meant not bringing them peace.
Once at the Lighthouse, he can be found on the balcony outside between the two statues, legs draped over the edge as he looks out into the Fade around them. Once the music room is available, he moves up to the balcony inside, just through the first portal, and puts a cot there for when he needs to rest. Sometimes, he hangs out in the music room, slowly teaching himself the piano and humming along. Somehow, the piano never went out of tune. He claims it's because the Fade liked the sound and kept it in tune this whole time. When not there, he might be with Lucanis, drinking way-too-sweet coffee.
He frequently hangs out with Harding, Lucanis, and Bellara. He particularly enjoys letting Bellara ramble about what she's doing; he knows it helps her work through her problems, and her enthusiasm is contagious.Â
His side quests take him through all maps, helping the poor and hungry. However, one time, he goes to a tavern in Minrathous and instigates a song for the patrons to sing to. It all leads to him needing help with some of Cole's trauma, and he takes Rook to where the real Cole's home used to be to leave flowers at his mother's and sister's graves. Depending on if Rook has hardened him to some extent and convinced him throughout his side missions not to help this or that person, he may request help defacing Cole's father's gravestone by knocking it over or scratching out the pretty words on the stone.
He's recruited from the beginning.
He's a third option for scouting in the second to last mission and can be killed in the fight against Ghilan'nain. His death leads to him returning to the Fade, however, not dying-dying. But he is never seen again in the game.
If he survives, he's best used to help take down the wards in the final battle by protecting the Veiljumpers.
He can be romanced, and there is a fade-to-black before the final battle. He frequently brings Rook flowers and small trinkets to decorate their room. It's unknown whether he's stealing the trinkets or not, though it's implied that he sometimes does.
He prefers peaceful options and talking through things over brute force, such as confronting the First Warden. He does not like releasing the Mayor, however.
He's for redeeming Solas, although he understands if Rook doesn't want to; it doesn't affect approval.
Of Rook's companions, he's the only one aware of Rook still seeing Varric, but he doesn't let on until after Rook realizes it, letting them have this to cope with the loss. He takes his death almost as hard as Rook does, and his depression is difficult to hide around them. He doesn't mask around the others, however.
He does not age and looks the same as he did in Inquisition, but his hair is much longer.