She wasn't invited as an event boss, but because she had control of the undead that were supposed to be invading cities around her territory and the devs didn't want to exclude those areas from the event. They also gave her the option of not participating at all.
There's been literally zero romantic chemistry between the two. The one 'ba-dump' he gets for Liz is resoundly squashed by him and Liz is more interested in Eleanor than anyone else.
I enjoy this version of otome isekai. There's another one similar to this: Chounin A wa Akuyaku Reijou wo Doushitemo Sukuitai ~Dobu to Sora to Koori no Himegimi