Lord Krishna considered Pandava queen Draupadi as his sister.
While beheading Shishupal (for punishment), lord Krishna cut his index finger (with his Sudarshan Chakra). Draupadi immediately tore off a piece of her sari and bandaged his cut. Krishna was immensely moved by this gesture of the queen. He promised her that that with this loving act, she has put him in a debt and he would repay each “thread” of debt when the time arrives.
And lord Krishna indeed kept his promise to Draupadi. When Draupadi needed Krishna’s help during her Cheer Haran (unclothing of Draupadi) as a result of Kauravas conspiracy against Pandavas, she fervently cried for his help. Lord Krishna eagerly came to her rescue and protected her honour by providing her with unlimited cloth which would never go off.
This is one of the famous stories of the origin of the Raksha Bandhan festival which the modern world greatly believes in.
In the epic Mahabharat, queen Draupadi had tied a Rakhi to lord Krishna, while queen Kunti had tied a Rakhi to her grandson Abhimanyu, before the epic war of Mahabharat began.