Lord there is so much wrong in this thread...

Terra's background is similar to Aerith's both are halflings of a superior magic imbued race that the main global power wants to use for their own devices but each takes a different view on how to come to terms with it. Aerith struggles to find more about her people so she learn more about herself while Terra literally struggles with discovering if she has any form of humanity and the fact her father's race were created as biological weapons.

Locke and Cloud are nothing alike but you'll discover that later, Celes and Tifa probably have more in common except Celes isn't a walking plot hole.

Vincent and Shadow are very similar though even though I feel Shadow has more of a reason to whine than Vincent and he handles his trauma much better than poor Vincent.

Kefka and Sephiroth are very different. Sephiroth is portrayed with an atmosphere of bad assness while Kefka ranges from comic relief to simply smurfed up disturbing. Both are narrcissistic with god complexes but they approach it differently. Sephiroth is methodical and does everything for the sake of his goal to become a god, while Kefka is more in the here and the now with no long reaching goals until the very final confrontation. Sephiroth kills to achieve his goals, Kefka kills for pleasure. In fact Kefka goes out of his way to kill people for the fun of it. Sephiroth only kills those who get in the way of his plans. Sephiroth believes in his delusions to the end while Kefka's time as a demi-god transforms his thinking to nihilism.

They're like opposite sides of the same coin...