The Ninja 's great. It seems to me that he could get extra points several times in the same turn, because when he ended up in a new location, he'd have to encounter any monsters there. This isn't necessarily unbalanced, although I suspect it's very powerful. I may try it out.
Katana is an amazing item - it's not unreasonable for him to have it, but if anyone else is ever going to be able to get it, it should probably cost $6 or even more.
Re: the Roboticist : I did test out the Rabbi I made, and tbh it wasn't that great. It's entertaining, but it's too rubbish at first and then too good later on.
I was thinking that the spawn should probably only to be able to move only 1-space-per-turn, except that there ought to be some way that the investigator character can cause it to 'move with him'.
"Any phase: Exhaust this item to cause Robot Minion to move 1 space on the black or white path. If this moves the Robot Minion into the same space as Edward Cianelli, refresh this card."
So during the movement phase, you can move yourself one space, have the robot follow you, move one space again, it follows you again, and so on, and so on. So if you're willing to escort the robot, it can move as far as you can (plus one space further). Of course it can still only move on black or white paths, so it can't follow you into locations - it has to 'wait outside', which I think is quite a funny idea and not totally inappropriate.
Also, 'toughness two or less' covers too many of the game's monsters.... like, about two thirds of them, including some really nasty stuff like Colours Out Of Space. The spawn monster would still be pretty good if it only killed Toughness 1 monsters, but I think we need to look for a better way of determining what monsters it can kill. You could expand the monster's movement ability to:
"When Robot Minion moves into a location which contains a monster with Toughness 2 or less that does not have Physical Resistance or Immunity, Edward Ciannelli defeats that monster."
That means it won't kill vampires, colours, hounds of tindalos, and a few other nasty supernatural things. However it does just add a whole bunch of long words. My rabbi character was called Jacow Loew because both those names are short, so when you have to include them on another card, they don't take up much room. I like that your guy is named after a guy who played mad scientists in Republic serials, but if you're gonna fit the 'phys resist' clause on to the monster, you probably need the guy to have a shorter name. Like, seriously short. At least shorten his first name to 'Ed'.