Takumi is overjoyed to see Ikoma alive, and both of are incredibly excited over the fact that Ikoma's steam gun works. This turns into a Funny Moment when Ikoma casually admits that he's been bitten, then back to Heartwarming when Takumi realizes that he shows no signs of infection and is still his friend.
After Ikoma saves the Kotetsujou, Takumi throws him a line off of the train and begs him to grab onto it. He knows his friend is still alive, despite all appearances.
When giving out rations in episode three, Kajika gives Shino an extra one for her unborn child. It turns into a Tearjerker when Shino is killed after she turns into a Kabane.
Episode Four's conclusion: after Ikoma and Mumei have saved the Kotetsujyo, Ayame declares that they will remain aboard, and to help feed them, she will provide her blood. Kurusu objects... though not out of Fantastic Racism this time, instead he's doing it because he knows Ayame doesn't have enough blood, meaning that he acknowledges Ikoma and Mumei as the saviors of the train. He then goes on to offer his as well, leading to Kajika and Takumi to voice their willingness to keep their friends fed, followed by Yukina, Kibito and many others in the engine to express their support for the once reviled Kabaneri.
And by episode 5, Ikoma is a valued member of the train's leadership, helping to build new bullets and helping in plans to evacuate refugees.
Building up in episode 5 and culminating in episode 6, Mumei became increasingly unsure of her own strength and necessity to the survival of the Kotetsujo, worries born from a conversation with a former ally of her brother and from her inability to stop the Black Smoke/Fused Colony. This led to Mumei being buried alive by a rock slide, while Ikoma (whom she also deemed weak) fought off a horde of Kabane. However, Ikoma's unwavering faith in her and his successful defense of her from the Kabane returned her confidence to her, which led to her successfully killing the Fused Colony. The little flying hug between her and Ikoma was just the icing on the rice cake for the girl who had gone through so much.
The entire latter half of Episode 7 qualifies. Ikoma encourages Mumei to have hope, and promises to make her human again one day, allowing her to live a normal life. Later we have the festival, with the entire main cast making optimistic wishes for the future and then enjoying the fireworks. Seeing everyone so happy after all that had happened is a wonderful breather before Biba's arc starts.
in a moment that is somehow both heartwarming and tearjerking, Mumei mumbling in her sleep the next morning implies that she has found somewhere where she is truly happy, and that it is an unfamiliar feeling to her.
In the Final, Ayame takes on board the survivors from the capital and even the remnants of Biba's forces while they are trying to escape, reasoning that they are all equal in their desire to survive and aren't a threat any more.
The crew of the Kotetsujo picking up Ikoma, Mumei and Kurusu before trying to escape, culminating in Mumei's utterly adorable smile◊ as she jumps aboard with them.
Mumei begging Ikoma to wake up after they leave the station. And due to an inexplicable Pet the Dog moment from Biba earlier, he actually does.
The Big Damn Kiss in The Battle of Unato. Mumei gets closer to Ikoma under the pretense of having him look at something, then while he's focused on it she leans in and gives a cute little kiss on the lips. It takes a moment for Ikoma to register what just happened, he still doesn't quite grasp what that means for them and their relationship, and Mumei is adorably flustered.
◊ as she jumps aboard with them.
