Who was the president of the U.S. after Nixon but before Ford ?

Nobody, Nixon was the 37th, Ford was the 38th.

You could have googled "US presidents" and found this on a hundred or more websites.

Richard Nixon was President. Spiro Agnew was Vice President. When Agnew resigned Gerald Ford was appointed Vice President as he was current Speaker of the House. Then Nixon resigned and Gerald Ford was then made President. There was no one else in between.

He is looking for someone to say, Alexander Haig.

I'm not going to say Alexander Haig. I kind of figured that one out but the history books keep saying Nixon then Ford and skipped Haig altogether. Maybe in 40 years when they rewrite the history books Haig will slip in... who knows, dumber things have happened when they go to rewrite the history books. I know good ole' Rocky (Rockefeller that is) was in there trying as hard as he could to be president, but somehow he was not able to pull it off. He sure got close though, but no cigar.

Gerald Ford was appointed Vice President as he was current Speaker of the House.

Not quite right. Ford was House Minority Leader, not speaker (the house was controlled by Democrats at that time). Ford was nominated by Nixon to be VP after Agnew resigned, and his appointment had to be approved by both houses of Congress, per the 25th amendment.

No one.

No one. Nixon, then ford.

Syndicated columnist Jerald Terhorst, also Ford's one-time press secretary said that Haig was essentially "acting president" during Nixon's final year in office, but Ford was definitely the next president after Nixon. I remember Haig more for saying "I'm in charge" after Reagan was shot... and Ford for bad golf shots. :D