Imworld, is another dimension inside the MC's body. The task of this function is to save the character (Female Hero) and MC harem (Maybe) to be there.
The MC can enter Imworld as he pleases, to invite others, the MC must have a close enough relationship with the person that the MC will invite and she/he must also be able to keep this Imworld secret, this is because the safety of the MC and the character that the MC "Summon" will depend on this function.
An example is when MC "Summon" Nana, then people who know MC know Imworld who can also enter Imworld at will, especially since he also has bad intentions to kidnap the "Loli", it is certain he only uses the MC for his own greed.
Therefore MCs really need to be on guard against everything, MCs must also not be easy to trust someone.
This is the "Taboo" that the MC must protect and keep secret. If this is known to the wider world, then bad things can happen to the character who is MC "Summon".