Joaquín Acosta Romero is the current head of the Buenos Aires Institute.
Joaquín and his mother were at the Buenos Aires Institute during the Dark War; his mother perished in the war, and the Institute was destroyed by the Endarkened. Clive Breakspear arrived at the Institute and took over as its head after the war ended, using the influence of his friend Horace Dearborn. When he found out that a Shadowhunter boy named Rafael was orphaned by the war, he tried to help him but quickly gave up when the boy resisted.
Joaquín was standing guard at the entrance to the Institute in 2012 when Alec Lightwood and Lily Chen arrived. He was horrified, but was unable to do much, when Breakspear turned away Alec's vampire companion, which also led Alec to leave and look for another place to stay with Lily.
When Alec returned to the Institute with Jem Carstairs and Rafael, he was standing guard outside again. Alec used the opportunity to try and recruit him in their efforts, warning him of the Institute's activities by giving him an address where Clive was set to do shady business with a local rogue warlock. Joaquín had his suspicions and, under Alec's instructions, brought a group of Shadowhunters he trusted with him to the address.
There, they found Clive and, despite their reluctance and fear of disobeying Clive, Joaquín and many others, urged on by Alec and his bravery, took a stand against Clive and his followers. They allied themselves with the werewolves that arrived, led by the supposed Queen of the Shadow Market, with whom Joaquín was immediately attracted to. He was inspired by Alec to be brave and noble enough to go back into the burning house for the captive werewolf women.
Alec called for a meeting after the encounter where he announced that Clive and his inner circle, to be named by Joaquín, would stand trial in Alicante for their crimes and that they should appoint a new leader for the Institute. Though Joaquín was quick to suggest that Alec take over, Alec was also quick to refuse and instead recommended Joaquín himself. Alec told Joaquín to work with the Buenos Aires Shadow Market in their efforts to rebuild. Joaquín immediately began flirting with Juliette and was pleased to find out that, despite having many adopted children, she did not have a partner.
Joining Livia's Watch
Joaquín attended Julian Blackthorn's war council with Juliette at the Los Angeles Institute in September that year. At the meeting after the war, he objected to giving members of the Cohort the right to vote for the next Consul.
Despite being young and obedient to any authority, Joaquin would step up and take the initiative when he witnesses injustice, such as when Clive Breakspear secretly organized for female werewolves to be kidnapped and experimented on. Unlike many other Shadowhunters, Joaquin shows sympathy for Downworlders and is not afraid to reach out a helping hand.
He also shows a shy side toward Juliette, and admiration toward Alec.