The person who is going to transform into Zenkaiser is called Kaito Goshikida. His name "Goshikida" contains the characters "五色" which means "5 colors", thus representing his suits.
The four Zenkaigers will have this untransformed form. Their names are Zyuran, Gaon, Bluen and Majean. When transform, they get the Zenkai added infront of their names. Guess this confirms that they won't be any actual human forms for these four.
Kaito's parents → Isao Goshida and Mitsuko Goshida have discovered the existence of Parallel Worlds. Currently they have gone missing. The names Isao & Mitsuko are taken from the singers Isao Sasaki and Mitsuko Horie who have sung the Theme Song of Himitsu Sentai Gorenger.
Yatsude Goshida is the grandfather of Kaito who owns and a Candy Cafe "Colorful". Kaito lives along with him. Of course his name is taken from Super Sentai's creator Saburo Yatsude (Saburo Hatte)
Secchan is a Mechanical Bird left behind by Kaito's parents. When the enemies began to invade earth, Secchan suddenly began to talk. His design is based on Baridorin from Gorenger.
Zenkaiser doesn't seem to have any Robos which he can use at the beginning. The other four Zenkaiger having their robo form, they can also transform into an animal / creature / vehicle.
Zenkai Zyuran → Zyuran Tyranno
Zenkai Gaon → Gaon Lion
Zenkai Majean → Majin Dragon
Zenkai Bluen → Bluen Dump
The four can form among themselves leading to different variations. Zyuran Tyranno + Gaon Lion = Zenkai-Oh ZyuraGaon Majin Dragon + Bluen Dump = Zenkai-Oh BlueMajin
These two are the major Gattai Formation. But sometimes they'll also use the following two variations just for a change. Gaon Lion + Bluen Dump = Zenkai-Oh BlueGaon Zyuran Tyranno + Majin Dragon = Zenkai-Oh ZyuraMajin.
Basically these Robos have their sides decided and a left sided robo can only form Gattai with right sided robo. So we'll never get to see the combination between Zyuran Tyranno + Bluen Dump & GaonLion + Majin Dragon.
Similar to Kamen Rider Decade, past Super Sentai exist in Parallel World for Zenkaigers. They will harness their abilities using the Sentai Gear along with Geartlinger an item created by Kaito's parents.
It's a Gatling Gun with the face of a bird. There will be a glass slot on the top which seems like it can be opened to place the Sentai Gear and then spin the gear using the roller on the right.
For now it's confirmed that we are going to Sentai Gears for Gorenger, Zyuranger, Shinkenger, Gokaiger, Kyoryuger, Zyuohger, Lupinranger and Kiramager.
Zyuran, Gaon, Majean and Bluen are from a Parallel World called Kikaitopia. They are the Machine Lifeforms "Kikainoid" of this world. Kikaitopia was a beautiful place for Kikainoids until it was taken over by an evil dynasty called "Tojitendo". They have spread fear in Kikaitopia with their rule and their politics.
Members of Tojitendo have their own set of Gears called Tojiru Gear. Tojiru means to close in Japanese. These small gear contain the special technology of parallel worlds and when set on a combatmen, a Kaijin will be born.