There are certain character types that are prone to bad luck, and mentor characters are firmly settled in that category. A mentor is anyone who serves as a pillar of guidance for the main character, so it could be the main characters parent, teacher, older sibling, and more. However, no matter what label this mentor has to the main character, after they pass on enough knowledge, something will remove them from the story.
One reason for this separation is as a writing trick to force character independence. If your main character has no one to guide them, there is no one to question their choices, no one to bail them out of trouble, and no one to hold them back. They can go where they want, and all the tough choices are now on their shoulders.
Example: a main character is traveling with and being protected by a gruff old man. He teaches the character along the way, and in the climax, the old man dies after protecting the MC. The man has done his job, and now it is up to the MC to take the lead. His death quite literally symbolizes the final step of transition for the main character to go from child to adult.
If you choose not to remove the mentor, it can be hard to come up with good reasons for why the mentor is not intervening when the main character is in danger, and can make it much harder to show that your main character is maturing.
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Media examples of killed-off mentors are: Uncle Ben (Spiderman), Obi-Wan Kenobi and Master Yoda (Star Wars), and Kelsier (Mistborn).
Another reason mentors are killed off is for character motivation. Sometimes a need for revenge fuels the character, while other times it is a need to complete a quest to honor the mentors memory. Very often, this mentor will die at the “first doorway of no return” plot point, which is when something big happens, making it so your character can’t go back to their life as it was before.
Example: The main character watches their mentor die, and is devastated. Later, their grief turns into rage, and they use that rage as motivation to get stronger so they can kill the murderer, and in doing so, avenge their mentor.
One warning I have about this plot point is that writers sometimes have the death happen, then the main character moves on from the grief unnaturally quickly. I am not saying your character needs to sulk the whole book, but having times that remind them of their lost mentor can create a burst of emotion in readers, and also show that the loss actually mattered to the main character.
Despite all the reasoning’s and ways to remove a mentor, it is absolutely possible to write both a helpful and present mentor, while also having a maturing main character.
Some examples of present mentors in media are Iroh from Avatar: The Last Airbender, Johanna from Hilda, Alfred from the Batman universe(when he is not killed off). You’ll notice that very often, though not always, there is some excuse for the mentor not being present when something very dangerous is happening. However, these stories still work to balance keeping the relationship with the mentor, while also continuing the growth of the character.
Other common options for removing the mentor:
They are captured by someone evil, and may escape in time to help for the climax, or may need saving.
They mysteriously disappear, and the main character does not know what happened. They mysteriously disappear, and the main character does not know what happened.
The mentor gets sick or injured, and the main character either has to look for a cure, or take on the role or rank the mentor once upheld.
In the chaos of a disaster, the main character and mentor are separated. The main character will have to continue on their quest while hoping the mentor finds them again.
Deciding their quest is theirs alone, the main character sneaks off without the mentor.
The main character must head off somewhere the mentor is not allowed to follow (such as an academy, or arena competition)
It turns out that the beloved mentor is working on an evil plot. Discovering the plot, the main character must leave to stop it.