Lord Guy is a noble in the Arling of Amaranthine.
Lord Guy will be present during the Oaths of Fealty quest at Vigil's Keep. He is standing at the brazier, in the company of Lady Morag. If the Warden-Commander is Orlesian, and they talk to him during the ceremony at Vigil's Keep, Lord Guy is intoxicated, and expresses discontent with what he considers an Orlesian attempt to rule Ferelden again while his father died trying to liberate the kingdom. The Commander can persuade him that forgiving one's enemies is the best way to give peace a chance, or use intimidation to make him stop talking.
If the Commander doesn't choose this dialogue or fails in the persuasion/intimidation check (Expert Coercion skill is needed), then he will be taken by the guards out of the Throne Room. Afterwards, the Commander will have to decide whether his drunken behavior should be ignored, punished or executed for his traitorous words.
Lord Guy is also part of the conspiracy against the Warden-Commmander and can be found and killed in the Old Stark's Farm.
Furthermore, if the Warden-Commander is Orlesian and while talking to him chose the persuasion/intimidation dialogue choices but failed or didn't choose them or Lord Guy wasn't spoken at all during the ceremony, he will assume a more important role in the conspiracy and even participate in And You, Esmerelle? quest.He will be responsible for instigating the peasants to start The Peasant Revolution (this happens only if Lord Guy is spoken but the persuasion/intimidation wasn't successful or wasn't chosen and A Brewing Conspiracy wasn't dealt with).