While Kaeya is The Lotus Princess, being shipped with Zhongli, I envision him wearing clothes like the ones you see below,

While Kaeya is The Lotus Princess, being shipped with Zhongli, I envision him wearing clothes like the ones you see below, from the Yume 100 otome game:

It would help to usher him into a complete 180° transformation, mentally and physically. In clothes of that nature, he definitely wouldn’t feel like a Knight of Favonius burdened by his past-he’d definitely feel more like a Princess experiencing a fairy tale romance. He’s either seen as a woman or someone that’s just as beautiful as a woman, neither of which he minds.

Kaeya’s outfits would be adorned with lotus flower charms, and his hair would be adorned with lotus petals. Similar to how Sonia from Pokemon Sword & Shield has hearts sprinkled through her hair:

The nickname ‘Lotus Princess’ comes from Zhongli’s associates overhearing the man-in-charge refer to Kaeya as 'lotus blossom’. That, combined with the Knight’s physical appearance and his visible relationship with Zhongli, causes the nickname to spread like wildfire.