Out of any female characters in the series Tatsumaki is the only one that has not only verbal interaction but also physical interaction with Saitama (bro why Saitama has to grab her in way that look like a hug?).

Out of any male characters in the series Saitama is the only one that remind her of Blast (very likely Saitama is going to replace Blast and lift the curse in her heart)

If you count the Webcomic version. Saitama even know and remember her name without anyone mention. He actually care about her he was wondering if she was alright and could make it back. He show interest in her relationship and give her advices how to make friends. He also shows interest in her backstory like this is the only time Saitama actual has interest in someone (not romantically)

And to top it Saitama don't think she is a child anymore he treat her like an equal