Lennon, who was on loan at Goodison Park last season, joins for an undisclosed fee on a three-year deal.
Mori, 24, who signed for £9.5m on a five-year deal, passed his medical last week at Goodison Park but had been waiting for a work permit.
Toffees boss Roberto Martinez said: "He (Lennon) had a major role in the second half of our season and we're looking forward to Aaron kicking on and continuing to enjoy his football, as well as being the productive player we know he can be.
"He is well-respected in the dressing room, his experience in the Premier League is immense and he will immediately add to our squad and to the ambition we have as a club."
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