I can answer this.
Items are for the most part unlocked via different aspects:
Story Mode completions
Tournaments
The Tower stuff
etc
The Tournaments for the most part is your main completion. The Tower will have dragonballs you slowly earn by having a secret fight come up.
The Character DLC packs are in the DLC character selection area. All they are are different aspects of the Story of characters for different strengths like Krillin before the Freeza saga, Gohan before/during Freeza saga as well as the Gohan after freeza saga, the different strengths of Fat Buu... etc. They always have the items and such associated with them whether you have access to them or not which is a plus in the sense that if you go online, you could use those if you don't have all the items you want.
In the end, they aren't extra characters.
EDIT: Apologies if I got the tower name wrong. I haven't played in a while mostly because I was doing the stars collecting and I am almost done. That is another thing I forgot to mention:
The only thing you technically can "Buy" is through star collecting. The First time you beat any story mission, you can re-do the mission with the newly accessed Star completions. 1 star is always consistent in that you have to beat it on "hard mode." The other 2 stars are acquired through different means and varies from mission to mission. Example:
Make a throw the first attack.
Do that at the start of the battle and you will acquire that star as long as you win the battle too. You don't have to get them all in one shot. Some are more difficult than others to acquire in one shot. Example:
Complete mission in Hard Mode
Complete all story attacks (Those things that pop up telling you to do a certain move at a certain time)
Do not use Ultimate Attacks
That set is usually a 2 times minimal to attain just because that the story attacks may usually consist in getting an Ultimate attack done. Sometimes this can be countered by doing enough damage before the ultimate attacks pop up. I think one mission was you couldn't do any energy attacks and the one under it you couldn't do Ultimate Attacks meaning if you wanted to do it all in one shot, any super that was just a physical rush attack would work, but other than that, you couldn't even use a ki shot.
