She has a fae heritage (as assumed by many of her talents and passives having a faerie theme to them), which means she has a long lifespan. In terms of lore, I imagine her to be some kind of witch who lives in Wolvendom, only appearing in the dead of night during cold winter periods. That way, going all out on CRIT Rate stats still provides a meaningful bonus as opposed to the traditional CRIT builds you see for DPS units (50 CRIT Rate to 100 CRIT DMG). To incentivize CRIT Rate further, her A4 Passive converts 40% of her CRIT Rate into Cryo DMG. If we assume Serene has 100% CRIT Rate (attainable on her through her base CRIT stats, plus signature weapon, plus CRIT Rate circlet), then the burst attacks deal an additional 61.9% of ATK as damage thanks to the conversion, on top of their existing multipliers. However to compensate, Serene takes her overinflated CRIT stats and converts a portion of it into motion value for her burst attacks. While she is in the Ourobouros state, she's locked in you cannot switch her out until the shield of her burst breaks, or the burst state ends. The buff allows her party members to build up her Serpent Scales, which will then be used to empower her burst (where she transforms into a kind of dragon for a short period). She is a Cryo Catalyst user who's role revolves around buffing the CRIT Rate and ATK of her party through her Lilac Frostbloom.

So, let's break down how this theoretical OC would work: I'm no artist (at least not in the digital sense I can draw traditionally but I haven't put pen to paper in that manner in over 6 years so rusty af), but having been a D&D brewer for a few years, I can draft up mechanics and designs in a text-based format, at least to a servicable degree. I had a thought experiment float around in my head for a few weeks in regards to Genshin Impact introducing a dedicated CRIT Rate scaling unit (Taking what Rosaria has and cranking it up to 11).

Genshin impact google doc build