Ryoma is often badgered by Kaito as to why he never tries to help the others with escaping. Ryoma only answers that he has no will to live, so he isn't interested in taking part of that. Kaito is upset by this, wondering why Ryoma is like this, remarking he was such an incredible tennis player and inspiring person back in the day. The two of them have very strong beliefs, Kaito being disappointed in the suicidal and depressive thoughts that Ryoma always has.
Being told to not get in the way of others surviving the Killing Game, Ryoma only scoffs, saying he'd never do anything uncool like that. When his corpse was found, Kaito seemed to express some regret towards being so cold like that to Ryoma, wishing he could've gotten through to him and give him a will to live. But he acknowledges it's too late to think about that now, and is determined to find his killer. He helps with the investigation by using his knowledge of tennis to estimate the net cord's length.
Ryoma thinks that Kaito is an easily manipulated person, as it was shown in a bonus scene where he was persuaded to fight him in a gambling match, in which he won in a landslide victory over Kaito. Despite the misgivings, Ryoma doesn't blame Kaito for being disappointed in him. Kaito used to be in a tennis club when he was younger, having looked up to Ryoma as an amazing tennis player himself.
Momoshi is not very popular in the fandom, due to their limited amount of interactions and their inclusion in other popular ships such as Harukaito, Oumota, Saimota, Tojoshi, and Goshi. However, it still garnered a fair amount of supporters.
The ship sailed mainly as a result of Kaito's worrying about Ryoma and his suicidal and depressive thoughts and regretting to leave him alone. It is 7th most written Ryoma ship fanfiction in Ao3.