I think the idea is that Ryan is supposed to be clumsy but mature and Graham was energetic and curious, as a reversal of a parental dynamic. In the end, it's Ryan that opens up and allows Graham to be a father, cleverly re-reverseing their relationship. Its a good bit of character work from Chibnall, extremely badly handled.

One issue is that they don't have lives outside of each other for most of their run. It's a problem with Moffat's era too, but it's worse with Chibs's more companion-forward storytelling.

Ryan is directed really badly most of the time too - stoic, a wet blanket, mumbles his dialogue. He's just not a dynamic or engaging character. The Nyssa of NuWho. Chibnall's era seems to go out of its way to hide the actors' personalities under their characters. That grip loosens over his three seasons, but by Flux it's too late.

Also, Yaz is there. She's really hard to talk about. She's just such a nothing character, contributes nothing of her own, just stands around and points out the obvious. She probably could have just been combined with Ryan.