so there's a ton of unanswered questions about hatter/takeru and his history with aguni in the show, but this is my personal headcanon for their timeline:
according to the manga, they met when they were teenagers and were hooligans together, running around getting into trouble
eventually, they brawl with the wrong (right?) guys and end up affiliated with the yakuza (also manga canon?)
anyways, after a little bit of that, one of their associates nabs takeru for the hosting gig. takeru takes it because uhh hello dream job??? get paid to flirt and drink??? so he goes off to do that and aguni gets kicked out of the organization for doing something stupid (getting arrested for arson or some shit) and decides to switch it up. joins the sdf.
they still stay buddies, but drift apart over time. aguni has to travel for assignments (he's one of the guys who makes the snow sculptures in hokkaido—yes, real, yes, done by sdf, and yes, he loves it) and takeru has an insane work schedule. eventually they don't really talk anymore, no beef just life getting in the way.
...until a training exercise goes wrong and aguni gets hurt. he gets sent back to tokyo to work a desk job that he actually kinda likes because it's stable and predictable. he decides to check in on takeru and finds out that not only is he one of the most famous hosts in the last decade, but he now owns the club he first started out at all those years ago.
it starts simple—grabbing coffee, catching up. takeru seems different, but so does aguni. they're not kids anymore.
it doesn't take long to figure out that takeru is an alcoholic. he doesn't hide it—calls it an "occupational hazard"—but he's losing control. if he's not passed out on the couch with a bottle of vodka, he's going through the emotional and physical rollercoaster of withdrawal. he lashes out. the business suffers. aguni tries to help, but he can only do so much. after a while, he stops answering takeru's calls.
a year goes by. aguni has settled into his new role, made friends, started to build a life for himself. he hasn't heard from takeru in ages, and that's probably for the best.
one day, he gets a text. a link to an obituary. takeru's father died. funeral details are included. aguni doesn't even question it—he puts on a suit that almost fits him through the shoulders and goes to pay his respects. takeru's dad was a good guy, and he treated aguni like an adopted son. it's only right to go.
of course, he sees takeru. it's a funeral, so not the best gadget of how someone's doing, but he seems worn out. the circles under takeru's eyes are darker than they ought to be, and he doesn't look well. too skinny, like something slithered by and sucked all the life out of him. aguni (herself, awkwardly) offers is condolences, and takeru accepts them graciously. takeru suggests he'd like to talk to aguni sometime soon, could they meet up in a few weeks? and maybe it's out of pity, but aguni accepts.
two months later, they meet in the park. aguni brings sandwiches from the convenience store (egg salad for him, one of those ones with the fruit in it for takeru) and they eat together. takeru explains everything; how his drinking ruined his life, how he was forced out of his job after his business partner died, how he ended the lease on his apartment and moved back above the hat shop to take care of his father during his last days. he's been sober for six weeks, and that's not a lot compared to other people, but it's the longest he's ever gone and he's proud of that.
aguni listens. what else is there to do? when takeru apologizes, he forgives him. aguni should stop by the shop sometime. it's the same as it always was, except takeru's at the helm (and they've started stocking fashion hats and sports team caps, something his father never wanted).
and that's where we get the flashback scene of them in the hat shop. takeru is healing. theyre reconnecting. sometimes friends really are forever. they start hanging out again. aguni is part of a baseball league and takeru starts coming to some of the games. they watch movies and fireworks and it's just like when they were younger, except things are better now. they're better now.
after they survive the first game in the borderlands, aguni doesn't even think about it. he grabs a bottle of bourbon from the deserted department store and takes a swig right from the bottle. he passes it to takeru without thinking, and takeru drinks, too. after everything they've been through, a bit of alcohol won't hurt. he needs something to take the edge off.
but "taking the edge off" turns into "help me sleep without nightmares" turns into "who even cares?" and takeru's fine until he isn't. they build the beach and devide to take opposing sides to keep the balance of power in check. it's not difficult for aguni to act frustrated with takeru, because he IS frustrated with takeru. they work together, but it's impossible for them to stay close.
and that's when we meet them at the beach and the story goes from there.
but basically i feel like the alcoholism/addiction thing that was mentioned in hatter's little monologue to arisu was a real thing he had to deal with. and he got better. he got out of his mess and was having a life...until the borderlands.
