- Underdog stories: people always root for the underdog. Falling from grace and hitting rock-bottom means that he can only go up, so readers can root for him as he works his way upward
- Humor is always an amazing tool to get readers on a character's side. There's a reason why people generally love the funny characters/characters that are having a lot of fun.
- Relatability: while I doubt readers can relate to being yeeted out of Heaven (unless I'm missing something lol), having him have struggles that can be compared to a normal reader is good. Having him be on Earth(?) after living in Heaven and struggling to make ends meet, get a job, find friends, etc--those are things that are very human struggles, so people can empathize and get on his side that way.
- Motivations give the character a goal, and help readers want to have the character to work to that goal. If he's working against an enemy (God???) that'd also be pretty epic, not gonna lie lol.
- Well-Developed Characters are key. Making characters three-dimensional, with passions, flaws, nuance, etc. is important to have readers learn more about them and get excited to be on their journey
- Convictions are also good--your character can be fighting for a noble cause or something.
- Be the enemy: because everyone loves Disney villains lol. They just have more fun. (If this is something your character's planning on getting into)
Tarra paused, thinking for a moment. "While we're at the gas station, I'm going to get a tootsie roll. Want one?"
Jann raised an eyebrow at her. "N-no. I didn't realize anyone ate those things anymore."
"Hmph! Well, I do." She stormed off into the store with overdramatic annoyance. A half-minute later, she had thrown the cashier six cents and walked out of the store, happy as could be as she chewed on her candy. "Mmmm..." she emoted, "You're really missing out, Jann!"
Jann didn't really know what to say, her friend looking too happy for her to put down without feeling guilty. "So...the gas is filled up."
"Then let's pedal to the metal!"
It's fun to make random scenes like this up.
I know people can get invested in inanimate objects(which obviously don't want anything), so it's clear this isn't necessary, however, as listed above my post, someone can get invested in something's humor, without any wants. I guess that's how harem anime get away with using protagonists as bland as such objects-humor, relatability, and underdog bias(kinda).
Many power fantasies tend to go with either the author self-insert or the blank slate (reader self-insert). The blank slate has as little personality as possible so as many people as possible can imagine being in their shoes. The author self-insert risks not being able to develop because the character 'can't' deviate from who the author is. This kind of character still generates more investment since we recognize parts of ourselves.
Personally, I went slightly different. I took traits that I recognize in myself and others and took a few to make the basis of each character. They're not all positive traits, I mix them up a bit. Good characters aren't perfect, they have flaws. Try to avoid caricatures at this stage.
Next comes the part where you make people root for them. Angry Raccoon gave a very good list on how to achieve that. I do want to add to the 'Be The Enemy' one - Everyone loves a good redemption story. Being booted from heaven for obvious flaws like arrogance, laziness and all-round trouble making only to be confronted with those wherever he ends up and starting to show that there's more to it, that there's a reason he got like that (how did I get this way?) and that he's working on changing himself around - slowly - and that there are qualities under the surface that are worthwhile...
The first step still remains. You need to present a character that is recognizable and relatable without being too simplistic
Also lots of other good advice here in the thread.
For A) shows us a bit of his life before the incident: perhaps his family or friends, his routine, his personality.
For B) give him a goal. Does he want revenge? A promotion? To live a carefree life? To get rich? To find a girlfriend? Whatever his motivation, establish it and justify it.
I Wrote: Save the cat???
"Listen, the hero has to come out with the cat. The cat doesn't have to be alive...." (from 'Going Postal')