Alessi wields the Stand of age reversal, Sethan. He is sadistic and cowardly and uses his Stand to gain the advantage before engaging his enemy.
Alessi fought Jotaro and Polnareff at Egypt. He reverted both Polnareff and Jotaro to baby age during their battle. Jotaro managed to retain his strength as a baby and beaten Alessi up with his bare fists.
Sethan manifests as Alessi's shadow. When a person stands in his shadow, his or her age will revert until s/he simply disappears from existence.