One year before the Saotomes arrived at the Tendo Dojo, Genma Saotome encountered Sankichi at a public shogi-playing club and the two had a game. Sankichi was on the verge of winning, but Genma distracted him and cheated by switching their boards around. Failing to realize what had happened, Sankichi believed he had legitimately lost to Genma, something that he found devastating.
Sankichi claims that he took to drinking to quell the shame of his loss, an act that drove his beloved wife Koharu (Clarabelle, in the dub) from him, but given his personality, it is unclear how much of this claim, if any of it, is true.
During the episode Shogi Showdown, Sankichi wanders into the Nerima district. He first drifts into a public shogi-playing club, where he loses to a teenager, who refuses to allow him a rematch, prompting Sankichi to use his Grandmaster Slam special attack to blow up the club. Then he tracks down Genma at the Tendo Dojo and demands a rematch. Whilst he finally gets a near-victory over Genma, the Saotome patriarch repeats history by defeating Sankichi with cheating.
Enraged, Sankichi demands that they play a match of Martial Arts Shogi. Despite the attempt by the entire Tendo Household to refuse, Sankichi finally gets his way by stealing the ingredients for dinner from Kasumi Tendo, leading to a chaotic match that ultimately comes down to Ranma Saotome vs. Sankichi, with the latter using Akane Tendo as a human shield.
The match ends mid-fight when Sankichi's wife suddenly arrives and demands he come home for dinner. Sankichi promptly slips out of his shogi piece costume and walks away with her, brushing off his battle as just "a game" and eager to go home with her.
To be added
Sankichi is an eccentric man of questionable mental stability. He attempts to portray himself as a calm, authoritative, manly figure, but is reality a melodramatic goofball who quickly resorts to begging and then violence if he doesn't get his way.
His greatest mental deficiency is his obsession with his wife; whenever Sankichi sees a girl, no matter how different they look to his round-cheeked matronly wife, he becomes convinced that they are his wife, and will attempt to embrace them, talking to them as if they are Koharu unless violently repelled. He will even kidnap women under this delusion.
Relating to this, Sankichi claims he is distraught over his wife having left him but when she arrives at the episode's end, it turns out that never happened at all. It's unclear if Sankichi was lying to play up his "dramatic" backstory, or if he's so genuinely delusional that he really had forgotten she hadn't run away on him. He instantly recognizes and falls in line with the real Koharu when she arrives, so the former seems more likely.
Sankichi is a competent practitioner of Martial Arts Shogi, although his skills at playing the regular tabletop version are distinctly less than impressive. He is followed by a loyal army of nameless goons, who dress in the traditional kabuki stagehand outfit as well as shogi piece costumes during his matches.
To be added
To be added