Semi Super Saiyan

The Semi Super Saiyan is a special Super Saiyan transformation which Goku Jr.

accidentally achieved when he tried to transform into a Super Saiyan 3 .

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When Goku Jr. fought against the revived Kid Buu, he knew 

he was not strong enough, he needed to transform.

Until then Goku Jr. had only achieved the first Super Saiyan transformation, he had no experience nor knowledge about the Super Saiyan transformations, he only knew they had more ascended transformations than just one. So he tried to collect al the Ki he could find and tried to transform into the long haired Super Saiyan he saw in his dreams (Super Saiyan 3). Goku Jr. did gather enough power to transform but something went wrong and he just transformed into a normal Super Saiyan, a normal one?

Appearance
Goku Jr's hair did have the form of a Super Saiyan but was green colored (a bit darker than Legendary Super Saiyan hair). Also his aura was not visible anymore, instead, he had red colored electricity 'zapping' around him and sometimes his hands created some ball shaped aura (image). His clothes also got light colored.

The Reason
The reason this all hapenned was because Goku Jr. did not have this much Saiyan roots anymore (since the last full Saiyan his family was Grandpa Goku himself). He can now be called a Half Blood Saiyan, this makes creatures with smaller Saiyan roots (starting at 1/16 Saiyan) need less effort to transform into a normal Super Saiyan but it's almost impossible for them to ascend the Super Saiyan form,.

This transformation WAS a mistake, but it is stronger as the Super Saiyan 3 itself.In this transformation, Goku Jr. can use stronger Ki blasts while he is still saving a lot of energy. So this transformation has a power level of at least 5.000.000.000(!)

The Semi Super Saiyan can use all of it's power in one final Ki blast (Goku Jr.s final Ki blast is called the Titanic Kamehameha) and then lose all it's ki and go back to normal

When Vegeta Jr. and Goku Jr. fuse, they are able to become Gogeta Jr. and then transform into a powerful Semi Super Saiyan.

On your right you can see the Big Bear Kamehameha which Goku Jr. used in his fight against Kid Buu.