comet, dark, obscure
孛 centers on a sudden, ominous celestial appearance: a comet, which brings darkness or obscurity. The meanings are linked by the idea of something bright yet foreboding that disrupts clarity.
孛 is a rare character whose historical origin is uncertain. It may have originally depicted a comet or a sudden appearance, but the component breakdown is not clearly established.
The top looks like a cross (十) over a cover (冖), and below is a child (子). Imagine a mysterious comet appearing in the sky, its tail like a dark veil covering the child, making everything obscure.
For ハイ, picture a comet so bright it makes everyone say 'Hi!' in awe, but its tail brings darkness: Hi -> ハイ.