His Ability to Act Like a Child
While the Twelfth Doctor could be very serious at times, there were moments when he acted like a child. Drinking in the chance to enter the Tardis and properly be fascinated by how it's bigger on the inside during the Doctor Who Christmas Special "The Husbands of River Song," was just one example of his childlike fascination.
The Doctor can sometimes act like he's the smartest guy in the room -- he often is -- but it's good to see a version fo the Doctor that can admit his mistakes. When The Doctor casually declares he's an idiot he becomes a little more relatable, and -- dare we say it -- just a little more human.
His Love for Clara
Clara and Twelve didn't have a romantic relationship, but it did feel like he respected her and felt a measure of love for her. Clara met her end on Trap Street in Doctor Who Season 9 Episode 10, "Face the Raven," but the Doctor wasn't willing to let her go so easily. He brought her back during Doctor Who Season 9 Episode 12, "Hell Bent," and refused to let her go, even when time wasn't healing itself. Now, that's the kind of friend that everyone deserves.
His Compassion for Bill
When the Doctor first meets Bill Potts in Doctor Who Season 10 Episode 1, "The Pilot," he is immediately enamored by her curiosity. But he also extends some compassion towards her when he finds out that she doesn't have any photos of her mother. If there was one aching regret we had, and the Doctor could rectify it with his Tardis and a camera, we'd hope he'd take a few minutes out of his day to do that for us.
His Faith in Missy
To say "it's complicated" between The Doctor and The Master is an understatement, but Twelve's arc with Missy is a particularly moving storyline. In Doctor Who Season 10, he saves her from being executed and makes a vow to guard her for 1,000 years in The Vault. While it's clear that Missy is a pain in his side, he never seems to lose faith in his lifelong friend, even when she meets her former self in Doctor Who Season 10 Episode 12, "The Doctor Falls."
His Wisdom Regarding War
One of The Twelfth Doctor's best moments will always be his war speech on Doctor Who Season 9 Episode 8, "The Zygone Inversion." The Twelfth Doctor is one of the first times in the new era of Doctor Who where we really saw a bit of his venom come out. In this speech about war, specifically, it feels like he's had enough of the human's pension for conflict and it's a very refreshing change in personality.