Temporary PC as a "boss" like character

I run a session for 4 static players, and a friend of ours recently had some time open up. I know he's interested in RPG's and would probably enjoy playing. That being said, I don't want to have to add a 5th static for scheduling reasons, and his schedule still is more hectic than the rest of ours(plus we've had some scheduling trouble recently).

I'm wondering if it would be fun to have him show up for a session or two here and there and play more of a boss like character who's an antagonist to the group.

Has anyone ever tried something like this? Are there any issues I'm not considering. This is my first time running an RPG in general so I'm pretty inexperienced.

That can work fine if he picks an appropriate playbook (Chopper is especially appropriate, though many others could also be good) and isn't horribly underpowered compared to the others (maybe give him a couple free advancements?), and he's cool with playing a character who's gonna die.

And, of course, assuming all the players are on-board with inter-PC conflict (which I'd assume most AW groups are, but it doesn't hurt to make sure).

What if he shows up, starts the random fight and wins?

I've had people playing villains in my games, but really, it didn't add any depth when they were basically a one-or-two shot thing as compared to just any npc. Playing as an antagonist only makes sense if he's human and doing it for real reasons, not just cause <enter super villain reason here>, and at that point he might as well just be playing a character like everyone else, and do really dynamic things at the start of the sessions he's in. Just be careful, sometimes the dynamic character with nothing to lose can steal the show, and when they leave, that show's void can be felt.

So, try it out and see, but be mindful of the consequences.

I would be a bit hesitant about doing this with a player who lacks familiarity with RPGs, as an MC who lacks familiarity with AW. That said, what I mean by 'this' is like, explicitly being like 'ok you're gonna play an Antagonist who is like, fictionally-special capital-A antagonistical and has special boss powers'. It just seems like a lot of risk for little benefit.

On the other hand, allowing the player to play a character more at odds / less ally-connected to the existing PCs is fine, because it won't cause as many problems as it would otherwise, and could add some fun spice -- but this doesn't mean they have to play that sort of character, either. There are plenty of other PC setups that could allow for a character to appear intermittently, or for a character to act recklessly and end up sidelined or act as a catalyst for the sessions when they are there. (For example, this may be the one and only time in the history of this board that I would actually suggest somebody play a Quarantine or a Touchstone.)

I'm not sure what the advantage of explicitly setting him up as an antagonist would be. Just let him create a character who's transient for some reason, and when he's there he's there, and when he's not he's not. If he ends up being antagonistic to the rest of the group that's fine, but he'll be missing out on a lot of the fun if you start out with him not having any opportunity for positive interaction with the other PCs.

Here's a thought - if your group is primarily in a single spot, like the Hardholders holding, get the occasional player to pick something mobile. If the team is mobile, get him to pick someone in charge of a place. Now he has a reason to not be around all the time. Ether he breezes in and out of town or they go to his place (or an NPC's place that he lives in/protects etc) from time to time for supplies, entertainment, whatever. He doesn't have to be an antagonist, per se. Look at the sample of play in the book. Each of the two players has a very different relationship with the NPC's. It's a source of drama and conflict without the two PCs having to go head to head. So maybe the Chopper isn't so much an enemy of the Hardholder but he does have a rowdy gang who tends to disrupt the peace, maybe even get in fights with the Hardholder's gang. Neither PC particularly wants to be the other's enemy or kill the other but there's going to be conflict that they have to resolve.