The word “hello” is not a verb, adjective, or (usually) a noun.
It’s an interjection, to greet or get attention. An independent emotional or social signal that, unlike other exclamations such as wow or ouch, can convey a wide range of emotions through tone, context and emphasis.
It can be friendly, upbeat, welcoming and positive. It can deliver neutral to warm affection. Elongated, or rising, it can deliver surprise or even astonishment. Say it sharply, to demand notice or show frustration. Or sternly to imply disbelief or irritation.
In writing, the effect can be varied with single exclamation mark (hello!) or double (hello!!), question mark (hello?), and with or without ellipsis, full stop or a combination of these.
Hello…?!!
You’re welcome.