I am writing an article. I want a noun that can be used to describe character of a person, usually a girl, that is too dependent on another person: always want to be by his side, always want to be together with him. In case of lovers, when they're not together she may call her boyfriend every hour, checking he's not with other girls, get jealous easily. Some people told me the adjective is 粘人 (nianren/zhanren). But they can't provide me with the noun. In English it is 'clingy' (adj.) and clinginess (noun). My sentence, should it be needed, is "Clinginess is not a favorable treat, yet many girls have it".

It depends on the context of your sentence. Could you please tell me the whole sentence that contains this word so that I can help you better?

Not every chinese word has n. v. adj. forms, just use it as an adjective. If you really have to use an noun:牛皮糖、小女人

粘人 as in lover who calls her boyfriend every hour, checking he's not with other girls, get jealous easily. 那就跟黏不黏人無關了 簡言之 就是 "妒婦" "醋罈子" 或 "愛爭風吃醋的女人"

會黏人的人嗎? 黏人精 神怪稱為「精」。如：「精怪」﹑「妖精」﹑「狐狸精」。 黏人精：擬人為怪 跟屁蟲 源自「跟屁股」，緊跟在後面。 官場現形記˙第六十回：「等到臬臺回到自己的衙門，首府縣跟屁股趕了來叩喜。」 牛皮糖 黏性、韌性都很強的糖。亦形容很會糾纏的人。 如：「他像塊牛皮糖一樣，甩都甩不開。」