Toffees boss David Moyes is well on the way to fulfilling his promise of bringing up to seven new faces to the club before Monday's transfer deadline.
Danish full-back Lars Jacobsen joined earlier in the week, before Ecuadorian midfielder Castillo completed his initial one-year loan from Red Star Belgrade after passing a medical on Thursday and receiving his work permit from the Home Office.
Castillo is still waiting for international clearance in order to line up alongside Jacobsen in the squad to face Pompey at Goodison Park this weekend.
And he is looking forward to making an impact in the English Premier League and European competition this season.
He told the club's official website: "I know that Everton are a big club who have a history of playing in important competitions like the UEFA Cup.
"I hope I can do my best for the team."
However the Toffees' second proposed signing on Thursday, Nigerian winger Obinna, will not be on board for the weekend after the club were refused a work permit for the Inter Milan star, who had agreed a one-year loan switch. An appeal will be lodged on Monday and it is hoped that will be successful.
It will still leave Moyes three short of his target with time running out, although there are reports suggesting Everton could offer up to £15million for Sporting Lisbon's Portuguese midfielder Joao Moutinho.
Rennes' Cameroon international Stephane M'Bia also remains a target, but the French outfit are standing firm in their determination not to sell him.
"We have received a good offer from Everton but we are not selling," Rennes general manager Pierre Dreossi said.
"We are going to seek a financial accord with Stephane so he can stay another year."