Seteth is a man who values order and discipline. He is very loyal to Rhea and the Church and shares her hostility towards those that oppose the Church or practice other faiths. He is skeptical of Rhea's decision to hire Byleth as a new professor. While he accepts her judgement, he initially expresses firm skepticism until they rescue Flayn. After that incident, he expresses complete gratitude in them and places his full trust in taking care of his sister in the meantime.
He dotes on his sister Flayn and worries about her constantly. He can be a bit overprotective at times, but ultimately loves her dearly.