Jacob Castell (Cross Guy) served as a reverend at a church in Sydney, Australia. After the Covenant attacked, he suffered from delusions of God speaking to him. Instead of evacuating with the rest of the citizens, he tried to crucify himself inside his church. However, he was physically unable to complete the task, and called for help after nailing one hand.
He was discovered by a small Marine unit led by Aliyah Burakgazi. They got him loose and then strapped him to a stretcher, where they could help heal him. They used the church as a fortification as Jiralhanae attacked. When the Jiralhanae broke through the Marines' defenses, Private Gagnon released him. However, instead of retreating, he ran at the Jiralhanae shouting, "Jesus will save you!" He took a spike through his throat within seconds.
Jacob Castell is based off of two "weird news" pieces. One was a man who nailed his hand to a crucifix, and then called 911. The emergency people couldn't determine whether he wanted medical assistance or for them to nail his other hand. The person who put up the article put "crossguy" in the web address. The second is a man who threw himself to the lions, screaming "Jesus will save you!"
His mumbled recitation of scripture ("…And I saw another sign in heaven, great and marvelous…") is from Revelation 15:1. The full line is "And I saw another sign in heaven, great and marvelous, seven angels having seven plagues, which are the last, for in them is finished the wrath of God."
"Castell" is a Spanish name originally used to indicate a person who lived near a castle. In Halo 3: Ascension, the castle referred to is the UNSC fortress Bravo-6.