naturally cheerful

[Itasan]

NEAKA
Literally 'root cheerful'. It is a slang
term used mostly by young people in Japan.
Are there similar slang words in English?
Cheerful not only now but it is his/her
born trait.
NEKURA
Literally 'root somber'. The opposite of the above.
Thank you.

[Tdol]

I don't know of a slang term, but we say 'disposition'

[M56]

NEAKA
Literally 'root cheerful'. It is a slang
term used mostly by young people in Japan.
Are there similar slang words in English?
Cheerful not only now but it is his/her
born trait.
NEKURA
Literally 'root somber'. The opposite of the above.
Thank you.

In casual use, you could say "he's born cheerful" or "born sombre".

[Itasan]

Thank you very much, everybody.