In the Extended continuity, Lion Heart is Kiros’ Gunbreaker weapon. It is considered distinct from the version used by Thancred. Lion Heart was originally owned by Yenke , an Ronso bounty hunter, and forged by Ramiun, a Drahn metalworker who also made Kiros’ Dragoon armor. Lion Heart scales with Kiros’ level and does not need to be replaced like normal weapons would.
In Endwalker, Kiros’ best friend and Skysteel Manufactory coworker Numair upgraded the Lion Heart using refined adamantite as well as leftover materials from Cruise Chaser’s upgrades. The overhaul drastically changed Lion Heart’s appearance, most notably granting it a lion-style grip and a blue energy blade imbued with Dynamis. The gunblade is thus called Lion Heart Prime. In Patch 6.5, the two show Yenke the remodeled gunblade, which he heartily approved of, declaring that it grew into its name.
In Patch 6.55, Numair can optionally show Lion Heart to Ramiun, who also approves of the modifications.
When using his Limit Break Blade Destroyer, Kiros wields both Lion Heart and Longinus as part of the attack sequence.