He is often said to be a spitting image of his father as he has dark blue fur the same colour as the flag of Uruguay. Compared to his sister Santiago does not wear as much clothing like the Russian vixen Natalya who wears a blouse and a pair of tights. His clothing choice is a light blue T shirt and a white nappy. In Argentina though he simply wears a nappy, a shell of even Natalya who also adds a skirt and heels, it is said that his mother tries to get him to wear more clothing but he refused however during his first time in Argentina he wears a nappy but his mother makes him wear an infant bodysuit which he accepts. In the Illusionist he simply wears a nappy. He often sucks a yellow dummy at times though it is much more common for him to suck a dummy than his sister.
Though he is older than Josefina Santiago acts like he is younger than her yet both of them act like babies because they are the only babies in the Junta. He and his sister have many parrellels with each other and the same goes for Maarja and Mirjana as they treat each other like sisters and so do Bogdana and Stanislava (even though they already are). He is also able to walk without aid most of the time and like his sister speaks "baby talk". However though Santiago is older than Josefina he is in fact less than a year old.
Compared to his much younger sister he is also much more babyish and fidgety, even though most of the Animalian Patriotic Front such as Saloth, Ne and a few others startle him he ends up startling Ali because of the fact when Ali first meets him Santiago fidgets whilst sucking a dummy which for unknown reason startles the Yemeni dog most likely because he never says anything and also his eyes are also very wide.
Much like his sister Santiago is very fidgety and likes to wander off but despite the fact he is a baby he can often walk unaided and has the gait of a child his age. He loves his sister and though boys go with Krypto Josefina brings him to Brainy who acts like a second mother to him (Lucia says) Like his sister Santiago is very playful but is very robophobic and it's thanks to his playful nature that many allies of the Junta namely those who wear leather, long clothing, latex etc. are very cautious around him and his sister, those cautious around him include: Chandrika, Asma, Faisal, Fahd and many others and also those who wear tights but even though his mother wears tights in Argentina he and sister do not play with them. In this case he is scared of the Daleks but his fear lies more with the Cybermen and the Autons. Even though he is older than Josefina he is also much more prone at times to tantrums but these mainly consist of him squirming, fidgeting, crying etc.
Not only is Santiago afraid of the Daleks, the Cybermen or the Autons but he is very easily scared of allies and enemies alike. In the case of his allies he is frightened of many Asian dogs including Babrak, Hafizullah, Saddam, Bikram, Dawood, Saloth, Nuon, Ne and many others but he is also frightened of Arnold especially on stage as the general throws a water bomb at him whilst doing an impression of the Green Goblin which leaves him with a wet nappy and he starts crying. The Goblin's voice is later adapted by Daniel in the dream world. As well as his mother Santiago is also comfortable around the Peruvian vixen Nadine and during Siad's trial sits on her knee.