We see Mihawk ate the Sheath Sheath Fruit: it lets you hide a Swordsman in your sword to increase its strength.
In a Stunning Double Page Spread, Shamrock returns to Mary Geoise, and gets shoved up inside of Yoru, while Imu and Mihawk are playing Dominoes.
Mihawk: "Come on, Imu: you know I can't fight anyone without him in here. Stop stealing Shamrock, what if Vista shows up at Emptee Bluffs?"
Imu: "Stop bitching, Mihawk. If it wasn't for me and this Fruit, everyone would know you're weaker than Buggy."
Shamrock: "Can I come out of the sword? I have to go to the bathroom. Cerberus doesn't like cramped spaces."
Imu: "I wish Teach had killed your brother, Shamrock."
Mihawk: "D-Don't just say Blackbeard's name, like that! What if he can hear us with the Gura Gura no Mi sensing out Vibrations?"
Imu: "Wapol-ass Fraud..."