Yoko Ono?

It does seem to be weird. The other guys seemed to be used to it and barely even acknowledged her. So odd.

“Did she pull up a chair and sit next to him while he was on the toilet?”

Not far off. Apparently John was very possessive and didn’t even want Yoko going to the bathroom without him. They also had side-by-side toilets in their home. Not to put all the blame on John for this weird relationship, as Yoko seemed more than happy with it.

Like a Japanese horror flick, especially when she starts singing.

Apparently they'd made some sort of commitment to be together all the time, which reportedly weirded out the other Beatles early on, but by this point it had been going on for two years and they were obviously used to it. Even accompanying Yoko while she yowled and telling each other "... if they want to be together then fine..." or whatever, when they were away from Lennon.

Which is a bizarre relationship undoubtedly based on Lennon's overwhelming neediness, but Yoko seemed fine with it, fine to the point of spending all her time at his side instead of doing her own work. She'd been a highly respected artist when she met him, but once in this relationship she spent all her time with him and did little or nothing of her own. The fans like to assure each other that Yoko was a succubus who ruined Lennon's career... but actually, he ruined hers.

She was indeed always around.

Her caterwauling was to die for.