So-Sorree appears for a brief moment in EP001 when Whisper is talking about the Yo-kai Watch.
In EP030, Nate is Inspirited by this Yo-kai at the beginning of its segment, causing him to play pranks on his classmates and cause various mishaps, all the while apologizing insincerely. This results in his classmates getting angry at him at the end of school. After Nate reveals So-Sorree and chases him, he summons Jibanyan to deal with him, but Jibanyan doesn't want to fight and isn't bothered even after So-Sorree eats his chocolate. Nate summons Blazion in turn, who quickly becomes annoyed by So-Sorree's insincerity. Nate has Blazion burn So-Sorree with his fiery mane, causing So-Sorree to fly around in pain before putting himself out in the river as Nate, Whisper, and Blazion mimic So-Sorree's insincere apologizing, giving the latter a taste of his own medicine. Presumably, after this encounter, So-Sorree surrenders his Yo-kai Medal to Nate.
Nate summons So-Sorree in EP032 as a last resort, having failed to start on his summer homework as a result of Lie-in Heart's interference. Predictably, his insincere apology under So-Sorree's Inspiriting fails to convince his teacher.
So-Sorree is briefly alluded to in EP072 after Nate encounters Bowminos, in which Nate reminds Whisper of another Yo-kai who caused its victims to apologize "forty-two episodes ago".
When freed from Crank-a-kai:"Oh, I'm SO terribly sorry you ended up with me. HowEVER can I repay you?"
Ittangomen is a wordplay on ittan-momen and gomen (ごめん, "sorry")."So-Sorree" is a corruption of the phrase "so sorry".
In French, Trodésolé comes from trop désolé (so sorry).
In Spanish, Losiento and Lo-siento comes from the phrase "Lo siento" (I am sorry).
So-Sorree is based on the ittan momen (Japanese: 一反木綿,ittan momen "one tan of cotton"), a tsukumogami from Kagoshima born from a bolt of cotton. It floats around riding the wind at night and usually appears harmless, enough that people may mistake it for a bedsheet the wind blew off of a laundry pole: however, sometimes it will descend on passersby, wrapping its cloth body around their heads to suffocate them. As such, it is quite more dangerous than the average tsukumogami.
So-Sorree was one of the two winning submissions of the first Yo-kai Contest, the other being Dracunyan.