There's this amazing thing that's known as the original IdleRPG :| It in every way tops mIRC scripting.

 on *:notice:login*:?:{
  if ($nick ison #idle-like-an-eagle) {
    if ($$2 == $readini(IRPG_Users.ini,$nick,Username) {
      if ($$3 == $readini(IRPG_Users.ini,$nick,Password) {
        set %irpg_ $+ $nick $nick
        /timerirpg $+ $nick 0 1 /irpgtimer
        writeini IRPG_Users.ini $nick Logged_On? Yes
        notice $nick Logon successful!
        msg #idle-like-an-eagle $nick $+ , the level $readini(IRPG_Users,$nick,level) $readini(IRPG_Users,$nick,Class) $+ , is now online from nickname $nick $+ . Reaches next level in $duration($readini(IRPG_Times.ini,$nick,Seconds),3) $+ .
      }
      else { notice $nick That password is wrong. Please try again.
      }
    }
    else That username is not registered, to register type /msg Aerialbot register <character's name> <password> <character class>.
  }
}

Could and should be:

on *:notice:login*:?: {
 if ($nick ison #idle-like-an-eagle) {
 if (($2 != $readini(IRPG_Users.ini,$nick,Username) && ($3 != $readini(IRPG_Users,$nick,Password)) { notice $nick Incorrect Username/Password, try again | halt }
 set %irpg_ $+ $nick $nick
 .timerrpg $+ $nick 0 1.irpgtimer
writeini IRPG_Users.ini $nick Logged_On? Yes
notice $nick Logon Successful!
msg #idle-like-an-eagle $nick $+ , the level $readini(IRPG_Users,$nick,level) $readini(IRPG_users.ini,$nick,Class) $+ , is now online from nickname $nick $+ . $nick reaches next level in $duration($readini(IRPG_Times.ini,$nick,Seconds),3) $+ .
}

What do you have to spot if a username isn't registered? also

/msg Aerialbot register <character's name> <password> <class>

What if someone doesn't name their bot that?

Use:

/msg $me register <character's name> <password> <class>

Besides..

  else That username is not registered, to register type /msg Aerialbot register <character's name> <password> <character class>.
  }
}

Uhmm..

  else  { That username is not registered, to register type /msg Aerialbot register <character's name> <password> <character class>.
  }
}

tips his hat

--Editted-- reason: added stuff so i wouldn't double post.