Guillen's body was recovered at the weekend, the latest victim of mega-storm Harvey whose death brought the toll from last week's historic flooding in southeast Texas to approximately 60.
"Alonso Guillen put the needs of others above his own safety and died while trying to rescue people in need," Castro said.
News of Guillen's death galvanized immigration advocates, as the Trump administration announced it was rescinding DACA, the program protecting from deportation some 800,000 immigrants brought to the US as children.
"Texas's Dreamers were victims of the flood, first responders to the flood, and at least one -- Alonso Guillen of Lufkin -- was a volunteer who gave his life to rescue his fellow Texans from the flood," said Terri Burke of the American Civil Liberties Union.
Guillen and his friend Tomas Carreon, 25, were with a group of rescue volunteers, when they both fell from their boat and drowned in the fast-moving waters, according to US media reports.
The Harris County medical examiner's office said his body was recovered on Sunday.
His mother, who lives in Mexico awaiting the right to legally immigrate to the US, was allowed to travel for her son's funeral in Texas, The Washington Post reported.
"He liked helping people," Guillen's brother Jesus, a US citizen, told The Post.
Typical of many Dreamers, Guillen's family was of mixed immigration status. His youngest brother was deported from the US five years ago, but his father was a legal resident, The Post reported.
As news spread late last week that Trump was looking to rescind DACA, another Harvey rescuer also waded into the debate.
A Houston-area paramedic who spent days rescuing flood victims said he was able to legally work because of the deferral program.
"We're not doing anything but giving back, being model citizens," Jesus Contreras, 23, told MSNBC.