Being liked and looked up to by other people makes characters more sympathetic. IE Sherlock Holmes is a pretty nutty and sociopathic guy but we forgive him because him and Watson are such good friends.
Quirky features increases character sympathy.
Proactivity! This will vastly increase sympathy. When characters don’t just passively respond to the events but are rather willing to pass through the depths of hell to accomplish their goals, we like them more. IE The opening scene of Indiana Jones 1. This can be easy to forget so watch it.
Depth of character and backstory increases sympathy. (more on this in the next sections)
When you write characters who have jerk-like qualities, make sure you give them redeeming traits to increase sympathy. Flynn Ryder in Disney’s Tangled starts out as a jerk but we still like him because of his activity and thieving expertness as he progresses from jerk to nice guy.