I would like to ask how to write a side story and develop some side charters that will be loved by the readers? I would like to develop some sort of strong feelings of the readers towards side characters too.
Let's assume that in your main plot has a particular story goal or problem as well as a theme. One way to use a side story or subplot is to present a different look at the same theme or to explore the problem from a different angle. By giving the reader a chance to see the issues from different perspectives you can give a more complete exploration. For instance, if your theme is selfishness vs. generosity, you can use subplots to explore different situations where either selfishness or generosity lead to good or bad outcomes. Or if you were exploring an issue such as war, you could use subplots to tell the stories of various people involved in or affected in different ways by a war, giving a very well rounded view of it.
As for making characters the reader will love, that partly comes from creating characters the reader can empathize with because they can see themselves in the character. The character has some of the same emotions, frailties, doubts, that the reader shares - even if they wouldn't admit them to others. Readers love characters they find endearing - well intentioned characters with limitations. They also love characters who have some of the same desires most of us have and the strengths we all wish we had (often in addition to the weaknesses we know we have).
You can also provoke emotions by creating characters the reader recognizes as like people they have encountered. Maybe none of us has met someone exactly like Ebeneezer Scrooge, but most of us have encountered selfish people. Maybe we haven't met a teacher exactly like Dolores Umbridge, but we certainly have seen manipulative or controlling people.
Of course, it helps to know your audience. If you're writing for children, bear in mind that children tend to empathize with characters their own age and admire characters a little older.
According to Dramatica theory, women tend to empathize more with characters of either gender who are running out of options to solve their problem, while men tend to empathize more with male characters who are running out of time to solve their problem.