By simply tapping into Kurama's chakra, Naruto's body begins its initial transformation. His blue eyes become red and pupils become slitted. Following this, his nails and canine teeth grow longer and sharper and his blonde hair will grow longer and spikier, as it stands on end. Naruto's whisker marks will also widen and thicken, adding to his feral appearance. While he can still stand on his own two feet, Naruto can move at greater speeds by manoeuvring on all fours in a manner that looks similar to the Four Legs Technique used by the Inuzuka clan. Depending on how much of Kurama's chakra was unleashed, it was capable of glowing around his body in an untamed way, similar to fire. Until he achieved the Nine-Tails Chakra Mode, it was the only form which resembled flames. Furthermore, after combining his chakra with Kurama's, Naruto and Kurama can switch control of his body at will, causing Naruto's body to enter the initial form so that Kurama can speak through his body.
At first, Naruto could only reach this state through sheer rage or if his life was in imminent danger, but after training with Jiraiya, Naruto learned how to contact Kurama and demand some of its chakra. Even though this was the weakest state of his jinchūriki forms, it still increased Naruto's physical strength, and its healing rate was able to regenerate wounds that were beyond normal repair. This form also increased his speed to such levels that nothing short of a fully matured Sharingan was able to track his movements. He could also use the chakra as a shock wave, sometimes merely using a roar or thrusting his fist out to send a rush of energy at his opponent.
When Naruto encountered Nagato face to face, he was shown to be able to simultaneously fuse his initial and Sage Mode forms together. His eyes took on the combined appearance of a mixture of them both. The exact capabilities of that form are unknown, but it was enough to completely resist Nagato's control of his Chakra Disruption Blades when it impaled Naruto. You can read naruto manga at this manga reader site.
