~(ㄴ/은/는) 것, ~(ㄴ/은/는) + Noun/Pronoun.
There are some variations on the ~ㄴ 것, or the general ~ㄴ + Noun form, a little involved but very useful to know.
First, it depends on the word's role: verb or adjective.
Note that Korean adjectives are almost like verbs unlike the English counterpart.
They are close to verbs in form, but differ in places as they only represent a "state" as opposed to verbs' "action".
- adjective (예쁘다 example): 나는 예쁘다: I am pretty / 예쁜 여자아이: A pretty girl.
- verb (가다 example): 나는 학교에 간다: I go to school / 학교에 가는 햑생: A student (who is) going to school.
Both 예쁜 and 가는 above modify a noun.
~는 것 is just one case of this general form where the noun happens to be 것(thing).
The important thing is to note 예쁜, not 예쁘는, for 예쁘다, and 가는 for 가다: ~ㄴ vs ~는.
~는 form is possible only for verbs since it denotes an "ongoing action" which adjective by definition can't have. Verbs on the other hand can have the ~ㄴ form, as in 학교에 간 학생. In this case, it represents the past (or present perfect for a result) tense, meaning it is a student who has "gone" to school.
So, ~는 것 is for verbs only, and ~ㄴ 것 has a slightly different meaning between verbs and adjectives.
This is the first step in understanding this important form.
Your sentence 그 사람 너무 예뻐는 거 있어요 is structurally wrong.
그 사람 너무 예쁜 거 있어요 is structurally ok but still doesn't make sense.
You can compare these:
- 그 사람 너무 예뻐요: He is so pretty.
- 그 사람 너무 예쁜 것 같아요: I think he is so pretty. (것 = that ...)
- 그 사람 너무 예쁜 것(을) 가지고 있어요: He has something that is so pretty. (것 = thing).