Per adventure I discover my friend is a vampire as well as his entire family, I'd still leave it a secret. Because it's probably for a reason he decided to keep it a secret. Sometimes keeping a thing secret might just be why we are not hurt and the only means to stay safe and live a peaceful life. A lot would change, I'd be more careful, avoid staying out late with him and try not to encroach into things he warned me about . The changes may be noticeable to some degree but I'll try as much as I can to remain normal to keep the friendship we once shared.
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Sometimes not knowing certain secrets is better than finally knowing. Keeping the secret to myself would be like a thorn in my chest, I will be tempted to say it out, scared to associate myself with him like we used to, I may look at him differently, and probably scared to eat certain things the family offer but the safest way to live a peaceful life is holding it through the moment no matter how hard keeping the secret may be. It would be one thing that would remain uncovered by me because the surest way to handle the truth is to pretend like there was no truth outside the one you once lived.
We are friends, I believe he won't hurt me as long as he believes I don't know the truth. Secretly he may even be my defender, helping me stand against bullies and any form of threat, but while he can do that, I'll also see him as a suspect whenever any situation of evil is reported. Letting the cat out of the bag may seem like a way to stay safe from future threats, but it will only worsen the exact future that one hopes to protect. So why not just live in a world where vampires and humans cohabit, a world of pretense where one party suffers the burden of hidden secrets while trying to live a normal life. I'll leave someday to a distant place where I will live with no fear, but while I still live with my friend, what I've seen will remain a secret.