After karthikeya kills tarakasur in the battle, lord indra will give his daughter's hand in marriage to lord karthikeya. The daughter is none other than deivayani. After that, karthikeya will marry another girl valli (the daughter of a tribal chief - king). Then there will be misunderstanding and fight between the two ladies over their right on karthikeya. Then karthikeya will remind them that they were sisters in their previous birth and both had done sincere tapas and desired to marry him only. Since deivayani always prayed and meditated on karthikeya with sincere devotion in her previous birth, she became devraj indra's daughter . But valli on the other hand prayed sincerely to karthikeya only during prayer time (not always), so she became the tribal king's daughter.
The way karthikeya marry's valli is actually very romantic. He will go in the form of an old man before valli. He will ask for food and water saying that he is hungry. valli will take him home with all respect and give him food with all devotion. Then the old man will then catch valli's hand and tell her that he is in love with her and wishes to marry her. Valli will be shocked. She will tell the old man that in this age, this kind of behaviour doesn't suit him. But the old man will tell her that love doesn't have any age and that seeing her beauty, he fell in love with her.
Valli will get angry and walk out of the house. After a few minutes, a huge elephant will start chasing valli. Seeing the elephant, valli will be terrified and start running to save herself. Then she will come running and suddenly fall into the arms of the old man. The old man will carry her in his arms lovingly. The old man will take his original handsome form of lord karthikeya. Valli will be suprised. She will realise that her lord had played with her and she will start blushing in his arms. Then lord ganesh (who was actually the elephant who helped karthikeya in his romance) will bless his brother and sister-in-law and depart to kailash.