I think that Shiho can actually help Shinichi since Ran has the tendency to flinch whenever she sees a knife or a gun. While Shiho does the same, she can easily recover and can even use the gun well. She understands Shinichi to such a point that they can even communicate silently, knowing what the other already has in mind. In comparison though, Shiho more on relies on Shinichi's physical presence while Ran relies on his emotional presence. Although I want to tell Haibara in response to her line "I am just a shark looming around the ocean floor. What am I in comparison to the dolphin, the favorite of the sea?" that Shinichi is a detective, and detectives, "They can smell the faintest trace of blood. Their ears are even sharper, allowing them to hear frequencies as low as 40 hertz from 2 km away. They approach using their sense of smell, determine their prey's exact location with their electric fields, and attack. They head to the crime scene when they smell blood. They use all their senses to find the culprit. They tear into their prey and never let go, with the sharp teeth known as Evidence. That is a Detective." You can probably guess, that They are sharks. And by the law of science that Shiho must know well, only members of the same species may come together and mate. Since she is a shark, and Shinichi is a detective, who is a shark as well, doesn't that make them a perfect match? Even if the dolphin is the favorite of the sea, the dolphin and the shark are enemies and cannot come together. Thus, Shinichi and Shiho, are a perfect match. Such is my reasoning.