Promise

About what is this and what it has to do with Guilty Gear.
For those who like only canonical things: the person in the left is Ky Kiske in warm outdoor clothes. That's all safe information for you.
For "fan-fiction-tolerant" people: the person in the right is Justice. Or, to be exact, combination of Justice and Aria.
The picture is illustrating a piece of fan-fiction. After events in Arena and his talk to Ultimate Gear Justice Ky decides that he hadn't heard enough and gives her medical aid, before she is gone, gaining some time for her. They spend some nice time together, constantly arguing about Crusades, reasons, responsibilities and meaning of justice. Not long before her death Justice regains part of Aria's personality and thanks Ky for help.
The phrase is: "My silly billy knight...  Promise me... you will never forget why you fight".