This matches Leo very well as most lead protagonists wield a sword of some sort. The swords also represent Leo's sense of balance between strength, speed, and defense.
Mikey had used the katana swords once in both the episode "Pulverizer Returns" and The TMNT Adventure comic. With the swords in hand Mikey tried to act like the leader...which failed miserably; Splinter made him use the swords to teach the turtles to fight out of their element (though he nearly killed himself while handling it)
In season 5, Leo uses a new different type of katanas as his main weapon, since his old ones got destroyed by Kavaxas.
Leo's swords, whilst being referred to as "Katana", are actually "Ninjatō" (忍者刀 ninjatō), (also known as ninjaken (忍者剣) or shinobigatana (忍刀)).
Leo leads the pack, slashing out a path with glinting double blades of fury.