Nouns ending in -al, -el, -il, -ol, or -eul are usually masculine.
The whole family: -al, -el, -il, -ol, -eul. All masculine.
Is criminel masculine or feminine?
lecriminel
The feminine form is criminelle
The rule behind it
96.5% of -al / -el / -il / -ol / -eul nouns are masculine
In context
J'ai été suspecté d'être un criminel.
I was suspected to be the criminal.
Il parle comme s'il connaissait le criminel.
He speaks as if he knew the criminal.
Le criminel a laissé des traces de pas.
The criminal left footprints.