The Siayan and the Cat girl (Goku x Bagi) (DBZ x Osamu Tezuka)

In toad mountain

The next day, Goku and Bagi were summoned to the Hokage's office. Tsunade informed them that Jiraiya will be taking them to a secret location for special training. They were excited to see where they were going. Before they can even ask, they were immediately reverse summoned to Mount Myoboku. The home of the Toads. Goku and Bagi were stunned to see around them a bunch of toads seemingly large and intimidating. 

Goku: Wow! Jiraiya, you didn't tell me we were gonna be trained by frogs!

Gamabunta: Who are you calling a frog, Gaki?

Goku froze when he saw the Toad Boss in front of him. 

Gamabunta: We are toads. Not frogs. So learn your species, teme.

Goku: Sorry, Mr...

Gamabunta: Bunta. Gamabunta. I am the leader of the Toad clan. The Toad boss.

Bagi: Sorry. We were just taken aback by the way we were summoned. And the way you looked at us. 

Jiraiya: Ah don't worry. You two will do fine. The Toad boss here will teach how to become Sages and how to summon and use the power of the toads. 

Goku: Sage? What's a sage?

Gamabunta: The sages in this world are animal summoners who can use their summons to increase their chances of winning and increase their power by using sage mode.

Goku and Bagi: Sage mode?

Gamabunta: Sage mode is an empowered state that can entered when blending natural energy with one's chakra, creating senjutsu chakra. 

Goku: What kind of chakra is that?

Gamabunta: Senjutsu chakra, to a normal shinobi, can enhance one's abilities and can magnify their chakra reserves. The three components for Senjutsu are as follows: physical, spiritual, and natural energy. When in Sage mode, it allows the user to tap into the natural force of the world. It can also open up new techniques and powers up existing ones with this new chakra. 

Bagi: How can we learn Sage mode?

Gamabunta: It can be taught in two places. Here, and the cave of the snakes. The Trainee must posses extreme chakra levels in order to invoke senjutsu. It is also important to have their bodies be strong enough to support such power increase. 

Goku: Then how do we enter this sage mode. 

Gamabunta: It isn't easy to do so. The user must remain still in order to be one with nature and balance their natural energy with both their physical and spiritual energy. If you put too little natural energy into the senjutsu, it will not work. Too much natural energy and...

Goku: And what?

Gamabunta: You could be turned to stone forever. In vice versa, you could also be turned into the animal you learned the senjutsu from. 

Goku and Bagi winced at this statement, and so did Jiraiya. He did not like the thought of him being turned into a toad. 

Gamabunta: Luckily for you two, we have special tools to make your training easier. 

Goku and Bagi: Really?

Gamabunta: Some of these you might not like since one of them has to involve putting you in a special oil that will help you passively take in natural energy. This oil is used only for training purposes. 

Goku: Well...I guess that could work.

Bagi: We had training as weird as this back in our world.

Gamabunta: But we also have a special staff that when struck at a senjutsu user, will take away all the gathered natural energy from the said user that reverses the transformations before they can become permanent and the user turns to stone. 

Goku: I guess that makes sense.

Bagi: What can we do in Sage mode?

Gamabunta: When in sage mode, the user's strength, speed, stamina, reflexes, perception, and durability are enhanced. The user's ninjutsu, genjutsu, and taijutsu will also become more powerful as well. The user can even manipulate natural energy as well. For those who have mastered the sage technique, they can have the ability to turn the natural energy around them into an extension of their body to increase the reach of their attacks. The user even has a special ability to sense chakra around them and can sense chakra attacks without even seeing them. 

Goku: What about when facing those who can drain chakra?

Gamabunta: Now that is interesting. When the user faces such person, it can be fatal for the opponent if they absorbed too much chakra and will be turned to stone if they haven't had sage training in the past. But, the user can also use the absorption technique against them. However, there are some disadvantages to the sage mode as well.

Goku: Like?

Gamabunta: When in Sage mode, the senjutsu chakra cannot be replenished. Which means you can only be in Sage mode for a limited amount of time. 

Goku: Well that's a bummer. 

Gamabunta: Actually, these weaknesses can be compensated through various methods and techniques. We will teach you the sage art of the amphibian technique. If you two can train in using Sage mode continuously, then the time recharging and gathering natural energy to use Sage mode will be drastically reduced.

Goku: That's incredible!

Gamabunta: So how about it, kids?

Goku and Bagi: *Nodded to each other* We'll do it. 

Gamabunta: Good. Because you too are incredibly strong, you will be trained not just by me, but by the two Great Toad sages, Fukasaku, and his wife Shima. Both known by Jiraiya as Ma and Pa. 

Goku: Alright!

Gamabunta: But first, in order to be trained by us, you two must first sign the summoning contract. 

Bagi: Summoning contract?

Gamabunta: It so you can summon us in battle and be able have us when in Sage mode.

After they had signed the contract, their training to become Sages had begun.