i remember one arc (sentinel and guide) mc pretends to be adorable, naive and cute. mc has a system too (the system is so frightened of his host acting so cute lol.) the ml is super strong (an sss) but has an unstable spirit (?) mc then have a big egg. he shouted and run downstairs about him laying an egg. everyone laughed at him. (the egg is phoenix dragon? ml's spirit is a dragon.) the son of the world is a transmigrated there (he is a fighter in his prev. world) to a body that is weak and bullied by the original body mc wear. he also a system. the original storyline is mc bullied the the son of the world and requested a duel (i forgot why). mc is from a famous family and has doting family members. the mc becomes friends with the son of the world too. another arc, mc is a merman engaged to the ml. the original owner of the body is adopted to the duke fam. because their own son had gone missing. ml's mother came to mc right after he wears the body and propose to cancel the engagement then mc agrees. the mc has two older brothers, a doting one. the mc decides to go to school where the son of the world is. he is also acting adorable here. they became friends. mc's second brother has feelings for him. the ml that time is in coma and he saw mc when he opens his eyes and began to look for him. unfortunately for him the engagement is already cancelled. he started chasing the mc. the ml also discovered that the mc can have a feet. please help me find it.
From my notes Arc 7 (Chapter 88-101): criss-crossed Fish life (Interstellar - Mermaid, MC gave up ML after he entered coma) from Who Touched My Tail sounds like the mermaid one and Arc 4 (Chapter 40-56): The abandoned Princess (Interstellar/ABO, MC already married as princess but fated to be empress) potentially might be the intersteallr one you were thinking of? But I don’t recall it being a sentinel guide type of scrnario EDIT: https://www.novelupdates.com/series/who-touched-my-tail/