"死党" refers to a true friend that one can rely on, and they have common interests and hobbies. He/she is the best and the most intimate friend. Here are several ways to say "死党".
1. friendship goals 令旁人羡慕的深厚友情
Leo and Kate are the epitome of friendship goals.
xiǎo lǐ hé kǎi tè yōng yǒu zuì lìng rén xiàn mù de shēn hòu yǒu qíng
2. fast friends 挚友
The two of them had been fast friends since college.
tā liǎ cóng dà xué qǐ yī zhí shì zhì yǒu
3. birds of a feather 合拍的人，习惯、嗜好相同的人
He'll like Tony – they're birds of a feather.
tā huì xǐ huān tuō ní de ——tā men liǎng gè yǒu xǔ duō xiāng sì zhī chù
4. joined at the hip 死党
Those two are joined at the hip. They are always together.
tā liǎ jiù xiàng lián tǐ yīng sì de, zǒng shì yī kuài ér chū xiàn
5. be (as) thick as thieves 亲密无间，非常友好
I'm sure she tells Ruth everything we said – they're as thick as thieves, those two.
tā kěn dìng bǎ wǒ men shuō de huà dōu gēn lù sī shuō le, tā liǎ qīn mì wú jiān