When I roll up a character for a game with a starting situation I have little influence over I start with what narrative impact I want to do. From what I know of the campaign whete I want to drive it. After that how I want to accomplish that, very much including things such as party role/niche and class. Backstory is generally something that comes last when I try and figure out why the character wants to do those things.

When I do have influence over the starting situation mechanical considerations comes earlier. I can mold the campain to a character idea rather than fitting a character into a campaign.

I usually start with a very broad flavour concept, then figure out how to do it in mechanics, and use that to flesh out the details of the flavour.

There have been times when I’ve come up with a very detailed flavour first and then gone for mechanics later.

The only time I start with mechanics is with one-shots or other times I don’t particularly care about the character themselves.