The reasons Dabi is Shoto's eldest brother Toya since his origins are unknown. Also Dabi knows the game Shogi and hates fish.
1. He knew Shoto's and Endeavor's real names.
2. Dabi has blue flames which is stronger and hotter and a flashback in Chapter 202 reveals that Toya's Fire Quirk was stronger than Endeavor's Hellflame. The canon name of his quirk is still unknown.
3. Dabi's quirk is like Izuku with One for All as both quirks are too much for their bodies to handle which results in purple colored skin it could even happen to Shoto (even with his ice side) or Endeavor since blue fire is stronger than red-orange fire.
4. Dabi's eye's are full turquoise like Enji's and Shoto's left side and his face resembles young Enji too.
5. In Chapter 191, Dabi admits that he could never stopped thinking about Snatch's words implying that he empathizes with what it feels like to have a broken family and in Chapter 192, Natsuo mentions Endeavor was responsible for an unknown incident with Toya.
6. Dabi didn't want to tell his real name to Tomura.
7. Dabi is distant from the League/PLF since 1) he's the only Vanguard Action Squad member who's real name wasn't revealed during the Hideout Raid Arc and 2) unlike Tenko, Jin, Himiko and Shuichi, his origin's aren't revealed during the Meta Liberation Army Arc. Also in Chapter 191, Hawks is left in the shadows of darkness and Dabi is walking toward the shining doorway of light similar to All Might in Chapter 1/Episode 1.
Does this mean that the next arc or the next will focus on Dabi (if he's Toya Todoroki) and possibly the Todoroki family?
8. Also Shoto takes after Toya/Dabi (if they're the same person) physically and in more ways than one (exluding hair and eyes expect the left side of Shoto).
- Seen between the way they activate their quirks:
- Their eyes: Dabi's eye and Shoto's eye
- Both are kicked in the head in Chap.41/Ep.24 and Chap.87/Ep.47.
- They both do this, links to Images: neck rubbing, steam/smoke and mist venting and similar expressions.
- Both use long range combat with they're quirks, which can be erased by Shota Aizawa and they get caught by his binding cloth.