I've said this before, but I think Celine could have been excellent if she was in Three Houses. The idea behind her character is very solid on paper - she's someone who is both ruthless and willing to do whatever it takes to keep her people safe, and someone who's deeply anxious at her brother's poor health and the prospect of having to take up the mantle of ruling. It's a good setup for a potentially interesting conflict and in something such as Three Houses with much better worldbuilding and actual character drama you could use this to facilitate a lot of interesting supports revolving around political or ideological conflict.

It's completely let down by Engage's overarching writing flaws - Engage doesn't have the political conflict or the worldbuilding required to facilitate a conversation about Celine's more interesting character traits, and Engage's writing is so afraid of interpersonal conflict the game misses the opportunity to really interrogate her worldview in a similar manner as to how Hanneman interrogates Hubert in their support. In the absence of good worldbuilding there's only so many so many supports where she can talk about her approach to ruling and most of her supports end up focusing on her surface-level traits such as her obsession with tea as a result - as such, what could have been a potentially outstanding character ends up being just an okay one.