I received my Celebi from a friend, but I don't necessarily know how he got his (it's probably from a Gameshark or an Action Replay). But, there are no strange side effects, and the Celebi is quite strong. It is at Level 100 (along with a not-exact clone at 82 that has weaker attacks, thank you cloning abilities:) and it knows Ancientpower, Hyper Beam, Psychic, and Future Sight. Along with my Celebi (nicknamed Cella by my friend), I have a Level 100 (without cheating, or else my numerous friends who traded with me traded a Level 100 to me)...
Machamp, Bakufun (my Typhlosion, nicknamed it's Japanese translation), Dragonite, Lugia, Blissey (686 HP!), Crobat, Meganium, Jolteon, Ho-oh, Lapras, Charizard, Feraligatr (I love the story behing his name; the programmers couldn't fit an "o" between the "t" and "r" or some other letter),Zapdos, Politoed, Mewtwo (Miky), Articuno, another Ho-oh (cloning:), Mew, Moltres, Kabutops, another Kabutops, and a Cloyster.
There you have it. A total of 23 Level 100's. But, trust me, I have been beaten by other Level 100 teams. My friends and I were nuts about Pokemon Red, Blue, Yellow, Gold, and Silver. So, we were always competing and training. Thanks to much cooperation, dedication, and hard work that started after I learned that the attack Tail-whip did not inflict damage, I can reflect back on this game today. And don't even try asking me to restart and do it all over again.