Nouns ending in -al, -el, -il, -ol, or -eul are usually masculine.
The whole family: -al, -el, -il, -ol, -eul. All masculine.
Is seul masculine or feminine?
leseul
The feminine form is seule
The rule behind it
96.5% of -al / -el / -il / -ol / -eul nouns are masculine
In context
Je suis tout seul dans ce pays étranger.
I am all alone in a foreign country.
Oh, mais il est venu à Tokyo seul ?
Oh, but he did come to Tokyo alone.
Je ne voudrais pas aller là-bas tout seul.
I would rather not go there alone.