Sonja also attended the Orange Star military academy with Sami whom she often studied with, though this bit of backstory for her is not mentioned in the English translation.
When the Orange Star military went across Yellow Comet to get to Blue Moon's forces, Sonja's father assumed threat of invasion and declared war on them. After Kanbei's Imperial forces were driven back, he consulted Sonja, for advice. Sonja asks him about what the conditions of the battle are, only to discover her father does not know. She then shares her intelligence about the current state of the Orange Star forces, advising him that good intel is the key to winning a fight.
Learning that the Orange Star force isn't particularly large, Kanbei vows to crush them with superior numbers, not listening to Sonja when she tries to tell him anything else. His forces are then defeated again and he is forced to withdraw them.
Kanbei again consults Sonja for advice. Sonja first advises Kanbei to construct more bases, at which point Kanbei stops listening and goes to fight Orange Star once more. After Kanbei's Imperial forces are defeated yet again, Sonja informs him that Orange Star had never intended to attack Yellow Comet and scolded him for not ever letting her finish what she was telling him.
Sonja later attempts to unearth the true instigator of the war between the nations of Cosmo Land. She soon discovers that Black Hole were the cause of the war, having gotten the nations to declare war on Orange Star by creating a clone of Orange Star's Andy and having him lead attacks on the other nations in the name of Orange Star. However, right after she learns this, she is kidnapped by Black Hole's Commander-In-Chief Sturm.
The four Wars World nations then join together to lead a final attack against Sturm's Black Hole forces. After rescuing Sonja, the COs of the four nations manage to overcome and defeat Sturm, restoring peace to Cosmo Land.
Advance Wars 2: Black Hole Rising
When Sturm returned with four other Black Hole COs to conquer each of the Wars World nations, Sonja and her fellow Yellow Comet COs fought to repel the Black Hole invasion of Yellow Comet and later joined in on the final battle against Sturm's forces.
Advance Wars: Dual Strike
After the Black Hole army mounted a counterattack against the Allied Nations following the destruction of the Black Obelisk, Sonja arrived alongside Sami, Grit and Eagle to reinforce the Allied Nations forces.