NERDS IN LOVEEEE I have a lot of feelings about them, especially because like we always say, they have unexpectedly meshed wonderfully since we first started playing out their relationship.
Robin's thoughts are fun to delve into because One Piece as a canon is very NO ROMANCE EVER, in the sense that almost none of the characters ever explicitly show romantic interest. The author himself has said their first love is adventure!1 and there's no main series ship (WELL THERE'S A SHIP BUT IT'S NOT THAT KINDA SHIP harr harr) so with Robin being who she is, her own individual thoughts about it are a lot based on headcanons and how she is with some canon interactions.
So with Porthos, she probably never quite expected to connect with him as much as she did with how casual things started out with them. But learning about each others' experiences brought them really close, and then just in general... she really loves him as a person. So in a lot of ways it's not only that they've been through similar things, but that knowing more about each other has let her see sides of him she's really fallen for too. He has a kind of approach to situations that she loves, and his drive is admirable, not to mention how much he cares about learning which is such a big thing to her. The way that he's been able to support her and understand her when things have been difficult for her in game is also really significant for her feelings towards him--not a lot of people have gone to the lengths Porthos would for Robin, and she won't forget that easily. So he's really important to her and in turn she's protective towards him in her way too, she might not be in your face aggressive but she can be pretty scary to make mad so basically no one touch Porthos because Robin's joined the PROTECT PORTHOS brigade.
Anyway, my point is they love each other a lot and with their feelings opening up more especially lately, Robin's not as inclined anymore as only being vague or smiley cryptic about it. Which does not happen a whole lot with her relationships unless you're a crewmate basically so it's a big thing for her.
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I'll be predictable too and ask right back at you for Porthos' thoughts on Robin!