Back in the 90s, Atlus decided to get into the Fighting game craze with Power Instinct, known in Japan as the Goketsuji Ichizoku.
Featuring 8 playable characters, it played like a typical fighting game at the time.
One of the things that made the game stand out is the fact that one of the playable cast is an old woman, whom one of her attacks involved throwing her dentures at you as a projectile.
The game would of course get a sequel a year later.
The first I've known of this series was back in the days when I'd frequent my regular arcade. I'd rarely play it, but it was PI2, and the intro that stuck to me was Kurara/Clara transforming and showing off her bare ass at the screen. I was 'umm what?' at the time and never cared for it, but I did like how when you beat opponents you scribble on their faces during the victory quotes.
Power Instinct 2 introduced Magical girl Kurara/Clara and offered 3 more playable old-aged characters, along with a kid who was clothed in some ye olde traditional asian/japanese garb. It also featured an arabian character named Sahad.
PI2 got a rather poor PSX release, wherein the game LOADS data during a match if a character transforms.
It got an updated version released on Arcade as Gouketsuji Legends/Gaiden, which used a system similar to KoF's team mechanic and balanced stuff from PI2.
Oh and it is interesting to note that PI2 featured vocal themes for some characters instead of the typical music theme in most other fighting games-
The 3rd game in the series was titled 'Groove on Fight', and much like SF3 featured mostly a new cast with the only returning characters being the Gouketsuji sisters!
The game utilized a tag team mechanic which reminds me a little of Kizuna Encounter's.
Groove on Fight was released to the Saturn though only in Japan far as I know.
In 2003, the Power Instinct series would then be revived on the Neo Geo via Matrimelee (Matrimony Melee?), with its Japanese ver. still bearing the series title.
This game brought back majority of the cast from 1 and 2, and introduced a few new characters. I don't remember much of what I played of it sadly, but I found the combo system decent enough.
Matrimelee got an updated ver. released to Japanese Arcades in 2006 titled Shin Gōketsuji Ichizoku: Bonnō Kaihō', which added Angela Belti and Kinta Kokuin to the cast, brought back transformations, and of course balanced stuff from the first game.
Despite having similar backgrounds to the first game, it features a new story, so this is basically Matrimelee 2.
The PS2 Matrimelee game is based on Bonnou Kaihou and featured Bobby Ologun as the boss-
In 2009, a 3rd Matrimelee game was released (by my count wouldn't this be Power Instinct 6?) titled Gōketsuji Ichizoku Matsuri Senzo Kuyou, which added 5 extra characters, here's a playthrough of the game in case anyone wants to see what its like-
All things considred, Atlus appear to still own the license to the series since they were mainly the publisher despite Noise Factory making the Matrimelee games.
I personally wish there were classic compilation ports of the series as I would love to try them out on current consoles or at least Steam. If the vocal themes cause an issue, they could just have the instrumental versions instead.
Oh and as a final note, there was 1 spinoff game featuring Kurara/Clara released to Arcades and the Satrun, Purikura Daisakusen, which was an action shooter game like Commando
HG101 entry on the Power Instinct series
Power Instinct Wikia