Talk more about that kiss in Ciel Nosurge...
It's a brief bit in one of the last scenes with a pretty dumb storyline "justificaton". Minimal spoiler version is that Ion used it to pass something to the other girl but there's no real reason it couldn't have been written as a quick bit of magic or something except somebody at Gust wanting to have them kiss.
On the other hand the epilogue is mostly from the POV of the other girl and basically shows her side of major events throughout the story and how she fell in love with Ion. It's mostly heavy subtext but a lot about her makes more sense if she's romantically into Ion rather than a close friend.
I don't know exactly how they address it in Ar Nosurge but I'm pretty sure Ion ends up paired with the self-insert character. Who is supposed to be you, the player, with all details about you obscured because you're piloting a robot from another dimension. The Nosurges get... really meta.