Santos is a member of the Maquis cell led by Macias that Ro Laren infiltrates on Starfleet's orders in Preemptive Strike, the penultimate episode of The Next Generation.
Santos appears in Preemptive Strike. He is one of the fighters in a Maquis group operating from a settlement in the Demilitarised Zone, alongside Kalita and under the fatherly leadership of Macias.
When Ro arrives claiming to be a fugitive from Starfleet, Santos is one of those who take her in and test her story. He is present as she proves herself by stealing medical supplies from the Enterprise, and takes part in the planning of the cell's operations against the Cardassians.
After Macias is killed in a Cardassian attack on the settlement, Ro's loyalties shift for good, and she warns the Maquis away from the trap Starfleet has laid for them.