Since Mokuba is now available at the Gate now, you don't have to play this limited-time event to unlock Mokuba.
Mokuba Kaiba' skills
How to get
Monstermorph: Evolution Can be used each time your Life Points decrease by 1500. Once per turn, send 1 Monster on your side of the field (except a Token) to summon 1 random Monster (that can be Normal Summoned) from your deck whose attribute and type are same to and level is 1 higher than the tributed monster.
Heavy Starter If your Deck has 3 or more Level 5 or higher monsters with different names, you will have improved chance of having a Level 5 or higher monster in your starting hand.
Draw Sense: LIGHT Can be used each time your Life Points decrase by 1500. In the Draw Phase, instead of doing a normal draw, draw a random Light attribute monster.
KaibaCorp Bling Can be used each time your Life Points decrease by 1800. In the Draw Phase, instead of doing a normal draw, draw a random Prismatic or Glossy Card.
Monstermorph: De-Evolution Can be used each time your Life Points decrease by 1500. Once per turn, send 1 Monster on your side of the field (except a Token) to summon 1 random Monster (that can be Normal Summoned) from your deck whose attribute and type are same to and level is 1 lower than the tributed monster.
Mokuba is Kaba Seto’s younger brother who is totally devoted to his brother. Although he is not seen as a duelists and is frequently get kidnapped several times by his brother’s enemies, but his supporting roles is not less important than any other main characters. For his beloved brother’s honor, he won’t back down for any challenge.
In Duel Terminal
In the Yu-Gi-Oh Duel Terminal, a player can challenge against Mokuba Kaiba. He uses the following three cards at least.
I think this is Draw Sense (Tech Card), if you get what I mean. If you're keeping only one copy of any card, to use against a very specific opponent- this card that you dont ever want to see except for this matchup- this might be the way to get to that card. Examples: Kuribohs, Defensive Traps, Quickplays, Armored Zombie for instawin, etc. Not the best skill, but it can be used.
This brat's skills list is even worse than Yami Marik.. There's not much decks able to build around using Mokuba. Even his Monster Evolution revolves around many specific cards which are hard to get or need to draw from UR packs
For all still confused about skills, when you use a skill summon a monster, its neither a normal nor special summon. Its something special unknown to the original game of yugioh. Thus any summon negate, cant stop skills. Skills are unstoppable basically.