Personally, I'd call the person wise. Really depends on the Reason Why the person is observing others. A person gaining wisdom would most likely observe wisely, meaning the person being observed wouldn't even know, but not to the Extreme. But a person just excessively watching someone, like staring, would be uncomfortable...Basically your question is unanswerable, unless you're more Specific.
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A person who observes other people is commonly called an observer, an onlooker, or a spectator.
Because they are dating one person then go behind there back with another one that is why we call them cheaters.
A person who makes people happy can be referred to as a joy giver, happiness spreader, or simply a positive influence.
A person who laughs at others is typically referred to as a bully or a mocker. Their behavior can be hurtful and can negatively impact the well-being of those they are mocking.
You could call this person a "selfish immortal."
A person with a big ego can be called arrogant, conceited, or self-absorbed.