Why is Koro-sensei's sibling Itona not like him?


When I watch the anime I get excited by Koro-Sensei's new student, Itona. He
has many similarities with koro-sensei. So, my question is: why do they look
different? Koro-sensei is like an octopus and Itona is like a human.




Itona isn't a relation of Koro-sensei.

> He is the son of Horibe Electronics Factory's former president. After his
> father's company went bankrupt, Itona was abandoned by him which became a
> source of bullying towards him as well. One night, he was found by Shiro who
> told him that in his eye lies a tenacity for power and that he would help
> him achieve that power in order to grasp victory.

See more: <http://ansatsukyoshitsu.wikia.com/wiki/Itona_Horibe>

I presume the similarities lie in the tentacles. Itona's powers are
manufactured by scientists, wheras Koro-sensei is an octopus alien.

![enter image description here](https://i.stack.imgur.com/ktSNTm.jpg)




As @Toshinou Kyouko explained, Itona and Korosensei aren't actually related.

Koro-sensei was...

> ... originally the subject of human experimentation. His powers are the
> result of the experiments that were run on his own body.

Itona is considered being Koro-sensei's brother because...

> ... he received his tentacles from the same scientists who experimented on
> Koro-sensei. His powers are inherited from Koro-sensei.

* * *

As for why they don't look alike:

> Itona had his tentacles implanted into him. Those tentacles were most likely
> originally part of Koro-sensei's body after the latter turned into the
> octopus we now know and "love". Unlike Koro-sensei, his body didn't actually
> change.

