걔가 너 보고 싶다고 했어

The line, explained

The lead has been pretending not to care for six episodes. Then the best friend sits down next to him and says, quietly: 걔가 너 보고 싶다고 했어. 'She said she misses you.' Cue the run through the rain. This line uses Korean's way of reporting what someone said, -다고 하다. It is the grammar of messengers and go-betweens, and you will hear it constantly, because everyone is always passing along what someone else said.

걔가 is 'she' or 'he' as the subject. 걔 is a squeezed form of 그 아이, 'that kid', and it means a person both speakers know about. Its partners are 얘 (this person) and 쟤 (that person over there). It is casual, and it does not show gender, so context tells you whether it is 'he' or 'she'. 너 is 'you', the person being missed, with the object marker dropped.

보고 싶다고 is the quoted part. 보고 싶다 means 'to miss someone', literally 'to want to see'. To quote a statement, put it in its plain dictionary-style form and add -고. Because 싶다 behaves like an adjective, its plain form is just 보고 싶다, and the quote becomes 보고 싶다고, 'that she misses you'. Spoken, the ㅍ in 싶 becomes a clipped P: sip-da-go.

The plain form changes by word type. Adjectives and 있다/없다 stay as they are: 좋다고, 있다고. Action verbs add -ㄴ다 or -는다: 간다고 (that he's going), 먹는다고 (that she's eating). Nouns take -(이)라고: 친구라고, that it's a friend. For the past, use the past form: 갔다고. Finally, 했어 is the casual past of 하다, here meaning 'said'. Swap it for 했잖아 and you get 'I told you so'.

Say it gently, like news that might change everything: gyae-ga neo bo-go sip-da-go, then a soft hae-sseo. Now deliver other messages: 걔가 너 좋다고 했어, she said she likes you; 걔가 너 기다린다고 했어, she said she's waiting for you; 걔가 미안하다고 했어, she said she's sorry. Each could be the turning point of an episode. With -다고 하다, you are the one who gets to deliver it.