WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 188, "Awakening," now streaming on Crunchyroll.
Boruto Episode 188, "Awakening," has kicked the Vessel Recovery arc into the next gear as Kara desperately searches for Kawaki. Koji Kashin has already killed Ao for failing to take out Boruto's Team 7, but little did he know they'd stumble across Kawaki's body themselves. Rather than swoop in, though, Koji sits back and watches the boy test himself against Konoha's heroes.
This leaves Delta, having none of Koji's voyeurism, to send an Outer Member under her wing, Garo, to take Kawaki back to Jigen. Interestingly enough, while the episode does follow the manga, there's some key insight added that significantly changes Kara's latest assassin.
In Boruto Chapter 24, when Konoha's shinobi awakened Kawaki -- who was unconscious after fighting off Kara's robotic puppets -- it seemed like they'd scrap with each other. However, the battle was interrupted by Garo, a short man with cybernetic blasters for arms sent to beat Kawaki up and take him back to the boss. For some reason, he hated the Vessel and, as he talked down to Kawaki, Garo leaked intel about Kara wanting him to be a host for some unique project, much to the spying Koji and Delta's chagrin. This led to Kawaki attacking him, closing out the chapter.
The same happens in the anime, but Garo's hate for Kawaki is actually put into context, beginning with the start of the episode giving another look into Kawaki's upbringing in Jigen's lair. Kawaki appears to have been in a fight but, rather than face a new opponent, Garo appears in a friendly manner. He appears mostly whole and it's clear that this is Garo before Amado worked on him.
Garo reveals that he's a prototype Vessel as well, only losing an arm, as opposed to other test subjects that died. Beyond that, seeing as they both survived the experiments, they're unique, so Garo offers to take Kawaki as a protege and teach him about Kara. Obstinate, Kawaki refuses and insults the war veteran, which irritates Garo into attacking. He's vicious, wanting to see who's the better Vessel, but the fight only leads to Kawaki's Karma mark activating, blowing off Garo's chin.
This leaves Koji and Amado in awe of his raw power. Garo's out cold and as he appears with his breathing mask, later on, it's clear he was badly disfigured after the incident. In an earlier scene not present in the manga, Delta actually pulls him out of slaughtering a village to go on this mission, so it's obvious as to why he's so eager.
He wants revenge for losing his jaw, ergo this grudge and why he's mouthing off against Kawaki in the present. It was always odd why he'd be so angry towards the Vessel, hinting he might risk killing him and then lie to Jigen that it was an accident, but now we know it's an emotional, personal vendetta. It's not about a job; it's a bitter rivalry between the jealous Garo and Kawaki, the Perfect Vessel.
