Back in Fate/Apocrypha, Jeanne was a Servant summoned as a Ruler to investigate the Great Grail War, and tldring it, she fell in love with cardboard-kun here, aka Sieg here on the right.
Fast forward to FGO, you can summon both of them. And Jeanne has a swimsuit variant, shown here.
If I'm reading the Commentary right (google pls), then she's giving her signature on a doujin she made, and she didn't realize Sieg was right in front of her.