During Roxas's battle with Xion, she dons armor which changes a number of times throughout the fight. Her basic form is a bizarrely-shaped suit that is predominantly silver and black, with some dull red armor near her waist, gold on her boots, and orange armored gloves. Xion's helmet sports two black spikes that jut out horizontally on either side and a large, red Nobody symbol on the faceplate, part of which forms something of a horn on the top of the helmet. This form is based off of Sora's original appearance, with Sora's jacket, pants, and necklace, although Xion's pants are pink and have designs, and her necklace features a Nobody symbol instead of a crown.
In her first form, Xion has large, skeletal, colorful wings and a huge, orange and red, intricately-shaped sword. In her second form, Xion loses her wings and grows to be about twice the size of Roxas, summoning a large, scythe-like weapon vaguely resembling the one of the Chimaera's arms as a weapon.
For the third section of the battle, Xion transforms into a figure with four huge arms, decorated with elegant, gold, purple and red designs. She also has large blades instead of hands, resembling Kurt Zisa's own arms. Her main body is also slightly larger than normal during this part of the battle. In her final transformation, Xion grows to titanic size, about the same height as the Train Station tower. Her armor is now embroidered with numerous spikes and ribbons. She also gains two Keyblade-like, pink and silver weapons with the teeth in the shape of the Nobody symbol. Interestingly, this form resembles Sora's appearance in Kingdom Hearts II, now being mainly black and dark grey and having a similar design.
This mission is really simple. As there are many Heartless during this mission, you may get stuck fighting them during the search for Xion. Once you move a little further into the Tram Common Roxas sees Xion on top of a building and she runs. The player regains control of Roxas and is able to make chase. Xion only runs and Dodge Rolls, and will teleport to another place in the area after taking some hits. The system for when this missions ends is unclear.
The timing for Xion to go to the secluded area is just set at random. At the end, Xion will teleport herself to that secluded place and waits there. Once the player reaches that point, they are launched into a cutscene in which Xion and Axel fight. This mission ends pretty much by itself and then there is a small conversation with Axel at the end of this day.
When it comes to fighting Xion as a boss, things get a lot more complicated. In an attempt to absorb Roxas and become whole, Xion transforms into an armored being that looks much like Sora, resulting in a series of battles. All of the forms (except the Final) have an attack that forms a barrier around Xion and heals her HP. To prevent her from healing, hit her when she floats into the air so the move will be canceled. An attack that is common to most of her forms is if her combo attacks are parried, she will perform an immediate, unblockable counter attack.
The first battle with Xion takes place in the Bizarre Room of Wonderland. In this form, Xion has large, colorful wings and a huge, orange and red, intricately-shaped sword. Despite the wings, she spends half the time on the ground. She attacks with her usual ground combos. Xion sends a burst of energy to knock Roxas away as she enters flight mode, where she performs an aerial version of Sora's Sonic Blade. If the attack hits, she will strike again; if she misses or the attack is parried, she returns to the ground. When she rises into the air, make sure to attack her: if she isn't interrupted, she'll form a barrier that restores her HP.
The second battle takes place in Halloween Town, at the Manor Ruins. Xion, having lost her wings, now grows to be about twice the size of Roxas and summons a large, scythe-like weapon vaguely resembling the Chimaera arm. She creates circles of light to pull Roxas in, leaving him open for an attack. Xion's attacks mainly consist of hacking and slashing Roxas, similar to Marluxia's battle style.
She can also shoot beams of Light at Roxas, using Sora's Ragnarok. This attack can be avoided by Gliding or by running (as long as a level 2 Haste is equipped). Note that her Ragnarok is a nil-based attack, so equipping a ring that is resistant against nil-based attacks will decrease the damage, or if you have Nothing to Fear equipped, no damage will be done.
Xion's third battle takes place in the Lamp Chamber area of Agrabah. Xion transforms into a figure with four huge arms, decorated with elegant, gold, purple and red designs. She also has large blades instead of hands, resembling Kurt Zisa. Her main body is slightly larger than the previous form. Her fighting style is much more graceful and agile than the previous forms; involving many flips and spins that make it harder to hit her.
She has the same light circle ability from the last battle and attacks with powerful, controlled slashes from her four arms. Xion can also cover her blades in light and perform Sora's Ars Arcanum. Xion can also perform a drop bomb attack similar to the Stalwart Blade, Heat Saber, and other Heartless of its kind: she will leap off the ground and drop right on top of Roxas a few moments later, inflicting massive damage.
The final, cinematic battle with Xion takes place outside of the Train Station in Twilight Town. Xion transforms into a titanic giant, about the same height as the Train Station tower. Her armor is now very embroidered with spikes and ribbons, and appears similar to Sora in Kingdom Hearts II. She also now has two Keyblade-like, pink and silver weapons with the teeth in the shape of the Nobody symbol. During the battle, Xion can shoot blasts of light and trap Roxas in a sphere of darkness, affecting his movement.
She also uses heavy melee attacks with her weapons that sometimes emit fissures of light and can perform a very powerful attack where she makes a heavy slash upwards, followed by a huge eruption of light magic shooting up from the invisible floor. She can also use her attacks that will cause status effects on Roxas. Her most powerful attack is a form of Sora's Trinity Limit, which deals out nil elemental damage.
Xion's attacks in this form are easy to predict, including her two most powerful attacks. Which one she uses is dependent on the attack she uses before the vortex appears on the ground. If she shoots the blasts of light and the vortex appears, she'll join her Keyblades and fire a blast of light in the air that will cause beams of light to shoot down on random places around the area (an attack based off of the limit break Roxas uses while dual-wielding: Magic Hour).
This can be dodged by gliding at Xion, as few of the light beams will hit the area directly near Xion. If she creates the orb of darkness before the vortex appears, she will use Trinity Limit, which can be dodged by gliding up to one of the corners farthest to the east and west of Xion. The good thing about this form is that most of her attacks are blockable and she will not counterattack.
- During the battle against Xion: First Form, Roxas has been shrunk down despite not drinking the potion, and also grows back to normal size when he is transported to the next stage, again without drinking the potion. Xion also changes sizes in the same way without explanation.
- Vector to the Heavens, the music that plays during Xion's final form, is a mix of Musique pour la tristesse de Xion and Kairi.