The nature of the geographical relationship between the world of the humans and the locations occupied by those sometimes classified as 'monsters', 'fairies' and others meant that links and transits could be made as appropriate - and humans would not sense the shift.
Woo Hwi-chul was able to recognise some of those with affinities to the entities of the otherworld: Soo Bo-ri had hinted at some of their functions when asked: conduits to the Spiritual Realm and restoring order, or disrupting it. Some merely had potential roles - and there were those linked with particular entities. Occasionally Woo Hwi-chul wondered what it would be like to become a mentor: eventually he would arrange his earthly role so it could be practical.
When Woo Hwi-chul met the children who were particularly so attuned he would engage with them - perhaps one might be the reincarnation of his lost son - though Son Bo-ri would not discuss that subject.
When he had bound Son Oh-gong in the Marble Mountain Woo Hwi-chul had given him various items to keep safe and to amuse him, and had at various times sent a human to retrieve one or another, with varying results. Hopefully by the time Son Oh-gong was released he would have learnt at least part of his lesson.
One of those Woo Hwi-chul had not sent to the Marble Mountain had been one Kang Dae-sung: there had been too much of a negative potential about him. Though young there was already something dark about him - he assumed power would be his by right of family, and there was a sense of a secret thereof deliberately suppressed.
Then he encountered a little girl who recognised he was of the otherworld, and wished to do no mre than exchange her yellow umbrella for his, should she achieve what he requested. The idea amused him - and showed she had the right attitude and nature. She was unlikely to release Son Oh-gong before those of the Spiritual Realm had made arrangements for him to become a guardian over some mortal, in order to learn responsibility.