The episode where we get to find out about Hayama's mom is great. Since Hayama's dad stopped talking about her when Hayama was very young, he doesn't have any idea about what she was like. Now his dad wants Hayama to know more about her, but he's a little communication-challenged, so he asks his wife's best friend from high school to talk with Natsumi and Hayama (that in itself makes for an interesting plot).
Anyway, it turns out Hayama's mom is very similar to Sana-chan: friendly, cheerful, and energetic. And of course very talkative. Like Sana, she has a sort of inherent kindness and lack of pretense that makes her easy to like.
The main fact that's revealed about her is that she loves all the seasons because they each bring something new and different. If you can draw any sort of general conclusion about her personality from that, it's probably that she sees the good in things and embraces them for what they are.
We also see that she was very loving towards Natsumi and to the unborn Hayama (it's not hard to imagine her instead of Sana talking to Hayama with his head on her lap about how she wants him to live for her sake and be happy).
If you think about it, just like Sana is similar to Hayama's mom, Hayama is very similar to his dad. He's generally reserved and serious, and also sort of uptight when it comes to being hyper and expressing emotions. So it's not too far-fetched to think that Hayama's parents may have been sort of like Hayama and Sana at some point. ^_^
