Who is Tiny Mystery Todoroki? His face we suspiciously have never seen in the two panels he’s been in. Is he a fifth sibling (if Dabi is a Todoroki)? Or just a random other sibling, or Dabi himself? I don’t know if he is a random sibling or Dabi, but I think there’s very little to suggest there is a 5th sibling for a few reasons, though you can never rule it out. I also lean towards thinking he’s Dabi.
The character book officially identifies four Todoroki children, in a particular order:

I guess it comes down to how much you trust the guidebook. It could after all just be interpreted from their sizes, though frankly I think this portrays Fuyumi & Tiny Mystery Bro as about the same size so I would hope there was, like, not just speculation. If that’s the case, then we have an explicit ordering of siblings. Of course, it doesn’t preclude another sibling, but it also doesn’t suggest it. Endeavor could have of course gone out and married someone else before Rei, or cheated on her, but there’s nothing in the story to explicitly suggest such as of right now and evidence in the guidebook suggests otherwise.
What was revealed in chapter 187 supports what the guidebooks suggests.

Basically, the ages we have for Natsuo, Shouto (15-16?), and Fuyumi all suggest an age gap of 3-4 years. Which, probably not so coincidentally, is the age kids are noted to manifest their quirks for the first time.

This seems to suggest that Endeavor was waiting until the child was old enough to manifest a quirk before having another one. Like, suddenly they’re not good enough. Dad of the year right there (this is sarcasm).

There simply isn’t room for another sibling and then Shouto, and nothing seems to suggest that Shouto had a younger sibling. Again this pattern of age gaps might not actually be intended to be a pattern or Shouto could have a younger brother, but there’s nothing textually supporting that, and I think it makes sense in a twisted way for Endeavor stopped producing children like products once he got the quirks he wanted.
The brother we don’t know about, regardless of whom he is, is therefore probably ~25 or Fuyumi’s (older) twin. Despite being short and tiny. (Or maybe Natsuo is just freakishly tall.)
The only possible evidence I’ve seen of a fifth sibling is the naming convention. Fuyumi’s name contains the kanji for “winter” and Natsuo’s for “summer.” That’s led to theories that there might actually be five Todoroki siblings with four of them having a season theme and then Shouto (whose name does not contain a kanji for a season). However, again, I personally don’t think there’s any direct evidence for this when the seasons referenced for Natsuo and Fuyumi expressly connote summer-heat-fire and winter-cold-ice which are related to the Todoroki family quirks (regardless of which quirks they specifically have). It’d be different if one of their names was related to spring or autumn, but it isn’t, so for now I can’t quite buy this.
tl;dr: there’s nothing explicit in the text suggesting a fifth sibling. Of course, there’s nothing outright ruling it out either, but Endeavor only thinking of four kids again strongly suggests four to me.

Now for what we know and don’t know about Tiny Mystery Bro:
1) Unless there’s been a retcon (you never know but I’m gonna go with unlikely given how the manga isn’t even adapting the anime’s hair color retcon) he’s probably the oldest, at either 22 or ~25.
2) He’s also the shortest. I bet Endeavor would not have liked that.
3) Natsuo referenced another sibling being abused.
4) We’ve never seen his face, in contrast to the other siblings, which is something an author usually does when they want you to have questions. (aka, foreshadowing. Like the fact that Dabi’s name has also been kept secret and implied it’s important, whether or not these two mysteries coincide.)
5) No one character has mentioned Tiny Mystery Todoroki’s name, & if he were no one who mattered (like, I don’t think Fuyumi or Natsuo are gonna be characters with arcs of their own as much as I would love more of them) there’s not really a narrative point to keeping it hidden from us and BNHA is about as subtle as a flamethrower (pun intended) in its narrative, though it does tend to be coherent. He could be dead, of course, but again, I don’t think anything within the story expressly suggests this (everything seems to suggest Rei throwing water on Todoroki’s face thanks to Endeavor being an abusive husband/dad was The Incident in their family, not the death of a child, though again, anything is possible).
6) Might be minor but Natsuo’s “I’m always in touch with you” which MS translates as “I still call home from time to time!” might carry a certain implication–that someone, presumably Tiny Mystery Bro since the same chapter tells us Shouto writes letters regularly and Fuyumi visits, is not in touch with them. I don’t know Japanese so that’s entirely possible I’m wrong here but given the explicit mentions of how the other siblings keep in touch in 187, I’m suspicious.
Occam’s Razor suggests that Tiny Mystery Bro is the eldest, was abused (or took the neglect harder than Fuyumi & Natsuo, which is also possible since neglect is also abuse and can be just as painful, & it’s Endeavor’s fault even if the pain Tiny Mystery Brother suffered was through a lack of actions rather than direct action), and is supposed to be something we, the audience, wonder about, and possibly the family wonders about him too.
If Dabi is a Todoroki (I think he is) he is probably that strange third child we see. Either way, I think Tiny Mystery Brother is probably very important to the story.



































