Okay, so our main character is a workaholic with a demanding boss, who are both together when a bus crash kills them both. Sekai Saikyou No Kouei: Meikyuukoku No Shinjin Tansakusha Manga Something something isekai something something skill points something something adventurer’s guild something something dungeon exploration. Look, you’ve been here long enough to know how it goes. MangaGo
So, the demanding boss becomes a Valkyrie and the MC, not wanting to work under her, decides to go solo. He soon must choose a class that his character will work as, and decides on "rear guard". However, he soon finds that a rear guard is actually a fairly rare job - and one he may just be able to excel at.