Keechak was the brother-in-law of King Viraat and the brother of his wife Sudeshnaa. He was very mighty, so mighty that he could be killed only by four people - Balaraam, Bheem,  Shalya and Keechak himself. He is a very important character in Mahaabhaarat as his misbehavior with Draupadee created a lots of havoc.

Although Keechak did not live in Viraat Nagar but he used to manage it from his own kingdom, so King Viraat was the King only for name sake, actually he was ruling the country. Once he came to Viraat Nagar, his eyes fell on Draupadee and he asked his sister Sudeshnaa to send her to him. In spite of his sister's warning that it is not advisable for him to have contact with her, when he did not listen to her, she sent Draupadee to him. He misbehaved with her and she came to King Viraat for justice. But as he was not the proper king, he sat silent on this issue. Yudhishthir also asked her to go back to the inner apartments of the palace. Draupadee understood and went away from there.

Bheem was standing there nearby so he in rage he uprooted a tree. Yudhishthir saw this. He knew that Bheem was in rage. Lest they are recognized in their Agyaatvaas period an they they again have to go another exile, he said - "What this cook is doing here?" Bheem replied - "I came here to take dry wood for my kitchen." Yudhishthir said - "But you have uprooted a green tree, you cannot burn its wood in the kitchen. Go somewhere else to get your dry wood for your kitchen."

Draupadee reported this to Bheem. Bheem advised her to show her love to Keechak and invite him in the dance room in the night. She did that. Bheem was already in that room, the room was dark, Keechak could not see Bheem, he took Bheem as Draupadee and as he touched him he fought with him and killed him. When Sudeshnaa heard this, she got very sorry for him.

When this news reached to Duryodhan, he thought that he had broken Paandav's incognito period because Keechak could only be killed by four people - Balaraam, Bheem, Shalya and Keechak himself. Balaraam had not killed him, Shalya had not killed him, so there remained only Bheem. It means Paandav were living in Viraat's kingdom, but unfortunately Bheem took this step only after considering all these factors.