I want a word to describe person Y who adores person X. As in this case, person Y admire person X one-sided. So what would be the word to call person Y? A more specific context is when this person Y, a junior of person X, fell gravely ill and the cause is partially because person X been ignoring Y, so person X said something like: I had overlooked the sickness of (the junior who adores me)
Smitten/Infatuated/Obsessed might work.
Smitten and Infatuated could be similar to 'crush' which might transpire to a more serious and healthy admiration. Though these doesn't strongly emphasize the 'unrequited' dimension, they are often used in anecdotes which end in it.
Obsessed is the harshest and least healthy of the three.
If the requirement is: Y adores X but X dint notice due to the distant admiration then "I had overlooked the infatuation of X (over me)" could work. However, if X knew about the admiration and intentionally did not reciprocate and Y still fell ill 'obsessed' could suit better.
A few good examples for the usage of smitten under the context of love can be found here: https://www.vocabulary.com/dictionary/smitten
What's wrong with admirer?
someone who has a particular regard for someone or something.
"he was a great admirer of Mark Twain"
synonyms: fan, devotee, enthusiast, aficionado; supporter, adherent, follower, disciple
"a great admirer of Hemingway"
a man who is attracted to a particular woman or a woman who is attracted to a particular man.
"she's got a secret admirer"
All of those synonyms are at least a good place to start your search.
Perhaps this indicates stronger feelings than you are looking for, but I recommend looking to this question and the accepted answer.
"The term lovelorn addresses both the unrequited love and the emotional state of the person whose love is unrequited."
The Oxford Dictionary defines crush as:
A brief but intense infatuation for someone, especially someone unattainable:
The term crush is usually one sided towards a person who doesn't know about it.
I have a crush on the secretary.