Spain/Portugal

Spain/Portugal is an uncommon pairing involving the characters Spain and Portugal. They are usually referred as SpaPort  or PortSpa, based on Spain's first three letters and Portugal's first four letters.

Relationship
Spain and Portugal are neighbors(brothers, they both have their last name as carriedo) with a rather turbulent history. When Portugal was first mentioned back in 2006, it was mentioned that they don't see each-other eye to eye very much, with their relationship being compared to the one England and France have. Despite that, they are shown to chat easily with one another, with Portugal in particular teasing Spain in almost every scene they interact. According to Romano, Spain acts meeker around Portugal, and the latter claims that's because he knows many foolish stories about Spain that he can't say no to him.

They are also very similar physically, and when Seychelles needed to bring Portugal a letter from England, she couldn't tell the two apart. This is attributed to the fact that they share the same lineage.

From chapters 428 to 430 of Hetalia World ☆ Stars, their relationship got expanded upon. In those chapters, Spain got captured and restrained by Portugal, who, after reading that their relationship is on good terms on both countries' websites, decided to test Spain with a five question quiz to see if their relationship is truly good, threatening to pour olive oil on Spain in case he fails each question. Spain, who hates wasting olive oil, tries his best to complete the quiz and gets the first question right, but he fails the next three questions, getting olive oil thrown on him by Portugal. In the end, Spain ends up appreciating the experience of getting olive oil poured on him, deciding to create olive oil specifically for the body after it.

According to chapter 428 of Hetalia World ☆ Stars, they have known each other for more than one millenia, and have been on bad terms multiple times. In Hetalia☆Collezione, it is stated that they used to be close, but now their relationship is delicate.

Fan Speculations
Because of the complicated history the two countries share in real life, there are differing interpretations of their relationship. In both the western and eastern fandom, the two are commonly seen as brothers, since they're similar to one another in terms of appearance, are said to share the same lineage and because in real life the two countries are seen as "brothers" too. In fact, the pixiv tag for them is called イベリア兄弟 (iberiakyoudai), meaning Iberian Brothers. In the western fandom, however, fans who ship them don't usually interpret them as brothers, and if they're seen as siblings then their relationship is interpreted as platonic only.

In the eastern fandom, a common topic for fanart is the time when they were children under the Roman Empire. Another topic of interest is the Iberian Union that happened between 1580 and 1640, which some fans like to portray as a marriage between the two countries.