If I made lists for all the things he's good at or among the best at, I'd have many more lists just to incorporate all his talents.  Koiwai is like the ultimate best friend and support character.  He's an instinctual talent at reading his friends' inner most thoughts and acting on them whether the character is ready or whether they are even aware of the things they like or need and Koiwai provides with ease.  He'd be about the best matchmaker on any such ultimate list.  He can intuitively sense attractions and reluctance to act on them then 'arrange' things so it happens.  He's also one of the funniest characters ever in anime if not the singular funniest.

It can be vaguely argued that she's more than a side/support character and part of the hero's personal entourage, but many other characters fit that role better and more practically.  For the most part, pointing out support characters is tricky especially if they are like Erika, who makes a big splash on screen but she's effectively used as a catalyst to push other characters; push the story forward; cheer them up; or even as a distraction at times.  She's like the sport team version of a utility man.  She's also a great all around character herself.

It's almost hard to simply put a guy like this as just a support character, because he really is so much more and frankly, I could just as easily put him on almost any list, and he'd fit right in, but he does, at least for a time fill the role of a support character beautifully (even he might say it like that himself).  Qi is one of the all around greatest and coolest characters ever, and totally outshines everyone in the anime Kingdom but his own guy Tang, who is the only one to get close to his level and perhaps surpass at some point.  Qi teachs the hero how to fight, be a soldier, and be a general all in the most creative manner you might ever see in a story otherwise plagued by letdowns and erratical character changes that suck the story dry or confusing viewers as much as the tv show HEROES did or make once good characters turn crappy compared to their original for no reason.  Thus, Qi stands out in a story that turns crappy by season two because he got a reprieve and no chance to screw up his great character.

Oocho is tremendously cute, charming, humorous, and fun to watch her growing role in the series as she muscles her way into a leading character role presumably for the next season.  In other words, she was so damn good, the studio actually raised her rank from support to lead by season end.  That's saying something.  Oocho deserves it, because she's hands down the best girl in the entire series so far, and yes she outmatches even the lead female characters.

Normally, I'm not a fan of psychopathic serial killers, but Barry's candid charm, wit, humor, and straightforwardness are hard to match with any character and raises his stake.  He might be the coolest, most fun, and most important character in Full Metal other than the leads.  Honestly, he even outdoes Greed's personality a bit partly because it's so unexpected he'd be such a boon to the show.

Viktoriya is for the most part mellow and unassuming but her fan obsession with Tanya; her deep yet realistic background story; and her casual yet relevant admiring narration carries the story along as well as her critical analysis of the lead Tanya in a way that makes her more appealing as well by showing sides the viewers don't see or aren't aware of or just misunderstanding her in a way that has an unexpected overall positive effect irregardless of Tanya's intentions.  She reveals a more human side to a character that tries to act like a monster perhaps in spite of her small size and looks.  But Viktoriya wants the audience to know, Tanya isn't actually a monster.

Chiffon stands in a overly violent horrendous torture fest of women in trouble, tortured, or murder gruesomely.  She's one of the few characters that seemed to have a firm grasp on reality and it's dangers and yet capable of packing more punch than most even among the so called powerful characters.  She's like a mother or more so, the ultimate older sister, who always looks out for everyone.  Sadly, she's a rare highlight in an otherwise utterly horrible and abusive oriented anime geared at hating on women.  If Jack the Ripper made an anime, it wouldn't even be halfway what this one was to lady mistreatments.

Crowley is the Chris Sarandon cool vampire of anime.  He's also almost the lazy guy from Fast Times at Ridgemont High.  Crowley is easy going; powerful; unexpectedly loyal to a sadistic dupe; charming; and cunning at least in one on one fights.He could come off as the cool older brother you wish you had.

He's an abrasive, selfish, battle junkie, romantic immortal with an ultimate best friend perfection vibe.  Whatever you as his friends need, he'll find a way to do but if someone or something interferes when he's being counted on, better say your prayers if it's you that caused it.

Kabuto might be a bit controversial in Naruto based on his story/ plot, but he's also a driving force of solid bankable influence.  He's Harvey Dent of anime with respect to Naruto or you could call him the Pat Garret of anime which is kind of a similar character/ role as Harvey Dent as well.

I wish we'd see more of him.  His imposing nature and role in the story is accomplished much better in the manga than the anime, but he's perhaps the most op character in Birdy the Mighty and perhaps he'd be a strong candidate for the most OP character ever that wasn't the main character nor technically the main antagonist either ever to appear.