An idiom describing two people who are familiar with each other.

Friendship is deeper than brotherhood.

affectionate

A person's love is as deep as the sea.

A firm and intimate friendship

Send charcoal to your door on a snowy day-send charcoal in the snow

Potential heart

True love and true care

brothers

Serve your friends or others wholeheartedly.

Help each other through thick and thin.

Extremely sincere to others

Be with someone. be a life-and-death matter

follow each other like body and shadow ― be inseparable

Blood is thicker than water

Send charcoal to your door on a snowy day-send charcoal in the snow

Stick together like glue or paint-in love

When two people are in a humble position, help each other

cross a river in the same boat