灵魂伴侣（ling hun ban lv)

灵魂伴侣 知己 至交

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我觉得直译“灵魂伴侣”就很好啊， "红颜"跟"蓝颜"有一点暧昧成分。 "知己"又不够soulmate. I think the direct translation 灵魂伴侣 is good! 红颜/蓝颜 is more than a friend (ambiguous relationship, maybe lovers but they are not a couple or bf/gf) 知己 does not reach the degree of 'soulmate'

thank you all for your comments - really useful :)