You should join the Lucario IRC, and we'll help you along.
Go to globalgamers.net, select the "webchat" tab, then type in "Lucario" in the channels tab, and put whatever name you want in the name tab, and you'll enter the chat.
(WARNING: Lucario chat is somewhat more (im)mature than the boards. You should probably leave once pictures start being posted and the members begin talking about "yiff.")
We're usually on in the afternoons and late at night (by West Coast standards), usually all the way through the night. We're nocturnal for some reason.
We're always willing to help out a fellow newcario.
Anyways, secondaries for Lucario really depend on who you want to counter, whether you want to pick up multiple secondaries or not, and whether or not you feel comfortable on all of the game's stages.
If you want to counter everyone and never be at a true disadvantage to anyone or on any stage and only pick one character, then choose Meta Knight.
If you want to counter some of Lucario's harder matchups (Marth, Mr. Game and Watch on counterpicks[but possibly not needed, so probably should be a low priority], Meta Knight, King Dedede, Olimar on counterpicks[but possibly not needed, so probably should be a low priority], Wario, Meta Knight, and Snake, meaning the priority list would be MK, Snake, DDD, Marf), then you should look through the character boards and find someone who counters most of these characters. Sadly, you will not be able to cover everyone with a single character, though, so if you don't intend on having multiple secondaries, try to figure out and prioritize exactly who you want to counter.
Know that Meta Knight is probably the trickiest character to cover, and that you should probably make him a top priority when deciding. Know that many characters, even if they do not go even with Meta Knight, perform better than Lucario does against him, and some even are able to go even with/have a slight advantage when you factor in counterpick stages. I would recommend making Snake a priority, as well.
The good, solid secondary choices would probably be (underline = counter, italics = better than we do, but still disadvantaged, normal = even matchup, bold = considered to be one of if not the best counter for said character):
Snake (Meta Knight, Snake[Ditto], King Dedede, Marth, Wario, Mr. Game and Watch, pretty much anyone else you want)
Pikachu (Meta Knight, King Dedede, Snake, and Olimar),
Marth (Snake, Marth[Ditto], Wario, Olimar, Mr. Game and Watch, pretty much anyone else below top tier)
King Dedede (Meta Knight, Snake, King Dedede[Gay Ditto], Marth, Wario)
Zero Suit Samus (Meta Knight, Snake, Marth, King Dedede, Wario, Olimar)
Falco (Meta Knight, Snake, King Dedede, Olimar)
Donkey Kong (Meta Knight, Snake, Marth, GnW)
Personally, I recommend Snake as probably the best option due to being arguably the only true counter to Meta Knight (and doing great against Marth and alot of the other popular characters better than Lucario can to boot, so if you just plain don't like a matchup, you could probably try throwing Snake at it) and pretty much countering almost everybody except King Dedede, King Dedede as another good option due to being the absolute best counter for Snake, Marth, and Wario, and doing as well or slightly better than us against MK to boot, and dealing with many of the matchups that can give Lucario problems, but aren't bad enough to be considered counters (like Wolf or DK, who often waver between slight advantage them, slight advantage us, and neutral), but he's still weak against Olimar and GnW on their counterpick if you're worried about that, and Zero Suit Samus is a great choice if you feel like going even rather than just plain counterpicking the crap out of your opponent, as she seems to go pretty even with our biggest problems. DK is a fun character overall, and seems to go even against Meta Knight, Snake, AND Marth, so you might want to try him out as long as you keep him the hell away from any King Dededes and Warios.
The above only really applies if you were planning on secondarying only one character, meaning your characters of choice would be Lucario and _____. However, if you plan on secondarying multiple characters, then the doors of opportunity basically burst open. You can have one character to handle Snake and Marth, like ROB, and another to handle Meta Knight and King Dedede, like Falco. Hell, pick a single character to counter each of our problematic matchups if you like and if you think you can really handle it. King Dedede is notorious for having many bad (often horrible, but not to the extent of an infinite horrible, like him vs. DK/Mario/Luigi/Samus/Bowser) matchups, so you can take your pick for a counterpick and then decide later on how to deal with the other 3-5. As long as you end up covering everyone (or most everyone), if you can handle it, it doesn't matter really who you secondary as long as you make sure not to use someone against the wrong character (like attempting to choose DK against a supposed MK main then having them turn around and use Falco on you).
However, one last thing that you should try to always take into account when deciding secondaries for Lucario are counterpick stages. Lucario has bad stages, some of which can even turn a supposed neutral or advantageous matchup (Like say... Fox or Olimar, one of which is supposed our favor, the other is neutral on many neutral stages), into a horrible disadvantage (See Corneria). The bad stages are basically all of the small and tiny stages, and the stages in which we are unable to camp or otherwise hold our position with our strong defensive game. These are basically Corneria, Rainbow Cruise, Green Greens (if not banned), Halberd to a degree, Brinstar(Everything screws with aura sphere, ridiculously tiny, no room to camp), Lylat Cruise(tilting screws with Aura Sphere, many characters have better platform pressure, however you can recover from under the stage by curving Extremespeed and are in general unphazed by the tilting stage, meaning you also have a slight advantage here against some other characters). Also, beware of Frigate and Luigi's Mansion in certain matchups, as the sheer tinyness of Frigate becomes real apparent when you're dying stupidly early to Snake's Utilt and magical disappearing C4 and mines with the BS lighting, and the ceilings on Luigi's Mansion can be easily turned against you when you fight a competent Marth, Snake, Meta Knight, DK, Olimar, etc.
In conclusion, Ganon really isn't a good secondary if you want to be effective, sorry. Ganondorf is generally considered to be one of the worst characters in the game, many times thought of as second or third worst period. His poor speed, absolutely ******** recovery, pitiful range, and overall lack of options weigh him down tremendously, no matter how freaking powerful he is.
