Hanta Sero

THEORY #1:

He is a tree who gained a Quirk and sentience

(Kind of like what happened with Principal Nezu)

Personally am not a fan of this theory because I feel like his name and ambitions would be a little different or people would mention it more as jokes/jabs/or from flat-out astonishment.

But I'm not ruling it out.

THEORY #2:

His home faced the tragedy, he just lived through it.

He is from Kagoshima Prefecture, so using the vague location as a clue:

He could be from Yakushima:

(AKA, the island inspiring the forest in Princess Mononoke - could tie into his name translation "God of the Woods")

Has a vast forest with some of the oldest trees around

See: Yaku-sugi (Jōmon-sugi).

Has a lot of hot springs (not relevant, just happy for him)

Native with rhododendron

(See costume: if flowers are supposed to be rhododendron and not roses).

He could be from the Mangrove Primeval Forest on Amami Island:

Amami has the Kanoya Rose Gardens

(See costume: belt attachment is made of roses)

2nd largest forest of mangroves in Japan

(Lot of trees, water, and kayaking)

* Both areas are near active volcanoes and subject to typhoons:

Is tragedy related to typhoons, floods, volcanoes, or another type of natural disaster?

See driftwood compatibility: If ships/ water are involved, his wood Quirk could've allowed him to float to safety for several days, being enough hope in the face of tragedy to spark the documentary.

THEORY 4:

Pirates/ boat-related tragedy, also includes being marooned, held hostage, and gives him a newfound sense of justice against villains.

Buuut I feel like he would have maybe joined a mariner-related field if his hero origins involved pirates.

THEORY #5:

Wood was harvested for harsh winters without consent like a sort of body-farm situation that was eventually liberated.

Like if he had bad parents or his whole family was a part of some sort of servitude angle?

He only started counting his birthday/ age recently, so there are a number of reasons why he never celebrated it?

(A little too dark for me, and may be a little influenced by the DNA chambers Yamato Tenzo grew up with in Naruto.)

THEORY #6:

Abandoned in the woods and grew up as a little feral nature child.

Also plays into the "just started counting my age" angle.

THEORY #7:

Related to his appearance.

Shinji is very secretive of his appearance, mainly his mouth area that is constantly hidden by a mask or scarf. It's possible, he just likes his privacy, buuuut... theories anyway:

He could be made of wood all over, have branch/limb-like teeth that are unsettling to others, a nutcracker jaw, Ghibli Forest Spirit face, or have a nod to a skin condition like Epidermodysplasia that he was ridiculed for as a kid, so now keeps his face hidden?